the ultimate goal of this change is to get all code used in libc.a out
of dynlink.c, so that the dynamic linker code can be moved to its own
tree and object files in the src tree can all be shared between libc.a
and libc.so.

contrary to commit 89e149d2, big
endian arm does need the instruction bytes in big endian order. rather
than trying to use a special encoding that works as arm or thumb,
simply encode the simplest/canonical undefined instructions dependent
on whether __thumb__ is defined.

the .byte directive encodes a guaranteed-undefined instruction, the
same one Linux fills the kuser helper page with when it's disabled.
the udf mnemonic and and .insn directives are not supported by old
binutils versions, and larger-than-byte integer directives would
produce the wrong output on big-endian.

IP_BIND_ADDRESS_NO_PORT is a SOL_IP socket option to delay src port
allocation until connect in case src ip is set with bind(port=0).
new in linux v4.2, commit 90c337da1524863838658078ec34241f45d8394d

IPPROTO_MPLS protocol number for mpls over ip.
new in linux v4.2, commit 730fc4371333636a00fed32c587fc1e85c5367e2

TCP_CC_INFO is a new socket option to get congestion control info without
netlink (union tcp_cc_info is in linux/inet_diag.h kernel header).
linux commit 6e9250f59ef9efb932c84850cd221f22c2a03c4a

TCP_SAVE_SYN, TCP_SAVED_SYN socket options are for saving and getting the
SYN headers of passive connections in a server application.
linux commit cd8ae85299d54155702a56811b2e035e63064d3d

Add new tcpi_* fields to struct tcp_info implementing RFC4898 counters.
linux commit 2efd055c53c06b7e89c167c98069bab9afce7e59

new in linux 4.0 commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8,
used to update atime/mtime/ctime only in memory when possible.

new in linux 4.0 commit 0189197f441602acdca3f97750d392a895b778fd.

This was new in linux 3.5 in commit cf60af03ca4e71134206809ea892e49b92a88896,
needed for tcp fastopen feature (sending data in TCP SYN packet).

remove ifdefs for powerpc64.

this flag for strong access ordering was added in linux v2.6.27
commit aba46c5027cb59d98052231b36efcbbde9c77a1d

the definitions didn't match the linux uapi headers.

a_ll/a_sc inline asm used 64bit register operands (%0) instead of 32bit
ones (%w0), this at least broke a_and_64 (which always cleared the top
32bit, leaking memory in malloc).

aarch64 provides ll/sc variants with acquire/release memory order,
freeing us from the need to have full barriers both before and after
the ll/sc operation. previously they were not used because the a_cas
can fail without performing a_sc, in which case half of the barrier
would be omitted. instead, define a custom version of a_cas for
aarch64 which uses a_barrier explicitly when aborting the cas
operation. aside from cas, other operations built on top of ll/sc are
not affected since they never abort but rather loop until they
succeed.

a split ll/sc version of the pointer-sized a_cas_p is also introduced
using the same technique.

patch by Szabolcs Nagy.

the files that used to come from extra src dirs under the arch dir
have all been removed or moved to appropriate places under the main
src tree.

commit f3ddd173, the dynamic linker
bootstrap overhaul, silently disabled the definition of __fpscr_values
in this file since libc.so's copy of __fpscr_values now comes from
crt_arch.h, the same place the public definition in the main program's
crt1.o ultimately comes from. remove this file which is no longer in
use.

all such arch-specific translation units are being moved to
appropriate arch dirs under the main src tree.

previously powerpc had a_cas defined in terms of its native ll/sc
style operations, but all other atomics were defined in terms of
a_cas. instead define a_ll and a_sc so the compiler can generate
optimized versions of all the atomic ops and perform better inlining
of a_cas.

extracting the result of the sc (stwcx.) instruction is rather awkward
because it's natively stored in a condition flag, which is not
representable in inline asm. but even with this limitation the new
code still seems significantly better.

this commit mostly makes consistent things like spacing, function
ordering in atomic_arch.h, argument names, use of volatile, etc.
a_ctz_l was also removed from x86_64 since atomic.h provides it
automatically using a_ctz_64.

this commit mostly makes consistent things like spacing, function
ordering in atomic_arch.h, argument names, use of volatile, etc. the
fake 64-bit and/or atomics are also removed because the shared
atomic.h does a better job of implementing them; it avoids making two
atomic memory accesses when only one 32-bit half needs to be touched.

no major overhaul is needed or possible because x86 actually has
native versions of all the usual atomic operations, rather than using
ll/sc or needing cas loops.

this is possible with the new build system that allows src/*/$(ARCH)/*
files which do not shadow a file in the parent directory, and yields a
more logical organization. eventually it will be possible to remove
arch/*/src from the build system.

switch to ll/sc model so that new atomic.h can provide optimized
versions of all the atomic primitives without needing an ll/sc loop
written in asm for each one.

all isa levels which use ldrex/strex now use the inline ll/sc model
even if the type of barrier to use is not known until runtime (v6).
the cas model is only used for arm v5 and earlier, and it has been
optimized to make the call via inline asm with custom constraints
rather than as a C function call.

sh needs runtime-selected atomic backends since there are a number of
supported models that use non-forwards-compatible (non-smp-compatible)
atomic mechanisms. previously, the code paths for this were highly
inefficient since they involved C function calls with multiple
branches in the callee and heavy spills in the caller. the new code
performs calls the runtime-selected asm fragment from inline asm with
extremely minimal clobbers, rather than using a function call.

for the sh4a case where the atomic mechanism is known and there is no
forward-compatibility issue, the movli.l and movco.l instructions are
provided as a_ll and a_sc, allowing the new shared atomic.h to
generate efficient inline versions of all the basic atomic operations
without needing a cas loop.

rather than having each arch provide its own atomic.h, there is a new
shared atomic.h in src/internal which pulls arch-specific definitions
from arc/$(ARCH)/atomic_arch.h. the latter can be extremely minimal,
defining only a_cas or new ll/sc type primitives which the shared
atomic.h will use to construct everything else.

this commit avoids making heavy changes to the individual archs'
atomic implementations. definitions which are identical or
near-identical to what the new shared atomic.h would produce have been
removed, but otherwise the changes made are just hooking up the
arch-specific files to the new infrastructure. major changes to take
advantage of the new system will come in subsequent commits.

commit 2f853dd6 failed to change the
test for -include vis.h support to use $srcdir, so vis.h was always
disabled by configure for out-of-tree builds.

otherwise C declarations are included into preprocessed (.S) asm
source files, producing errors from the assembler.

the lib dir is automatically created if needed by the out-of-tree
build logic, and now that all generated files are in obj and lib,
deleting them is much simpler. using "rm -rf" is also more thorough,
as it picks up object files that were left around from source files
that no longer exist or which are no longer to be used because an
arch-specific replacement file was added or removed.

also clean up duplication of CFLAGS passing to assembler.

as of commit af21a82c, .sub files are
no longer in use. removing the makefile machinery to handle them not
only cleans up and simplifies the makefile, but also significantly
reduces make's startup time.

commit 2f853dd6 failed to replicate
the old makefile logic that caused arch/arm/src/arm/atomics.s to be
built. since this was the only .s file under arch/*/src, rather than
trying to reproduce the old logic, I'm just moving it up a level and
adjusting the glob pattern in the makefile to catch it. eventually
arch/*/src will probably be removed in favor of moving all these files
to appropriate src/*/$(ARCH) locations.