previously, shared library constructors were being called before
important internal things like the environment (extern char **environ)
and hwcap flags (needed for sjlj to work right with float on arm) were
initialized in __libc_start_main. rather than trying to have to
dynamic linker make sure this stuff all gets initialized right, I've
opted to just defer calling shared library constructors until after
the main program's entry point is reached. this also fixes the order
of ctors to be the exact reverse of dtors, which is a desirable
property and possibly even mandated by some languages.

the main practical effect of this change is that shared libraries
calling getenv from ctors will no longer fail.

these should have little/no practical impact but they're needed for
strict conformance.

if these are to be supported, they belong in the main stat.h, not
repeated for each arch.

the missing check did not affect the default profile, since it has
both _XOPEN_SOURCE and _BSD_SOURCE defined, but it did break programs
which explicitly define _BSD_SOURCE, causing it to be the only feature
test macro present.

these structures are purely for use by trace/debug tools and tools
working with core files. the definition of fpregset_t, which was
previously here, has been removed because it was wrong; fpregset_t
should be the type used in mcontext_t, not the type used in
ptrace/core stuff.

actually, the hard-coded name should be eliminated too, and replaced
by a search for the soname in the headers, but that can be done
separately later.

aside from microblaze, these should be roughly correct for all archs
now. some misc junk macros and typedefs are missing, which should
probably be added for max compatibility with trace/debug tools.

it should now really match the kernel. some of the removed padding
corresponded to the difference between user and kernel sigset_t. the
space at the end was redundant with the uc_mcontext member and seems
to have been added as a result of misunderstanding glibc's definition
versus the kernel's.

with these changes, the members/types of mcontext_t and related stuff
should closely match the glibc definitions. unlike glibc, however, the
definitions here avoid using typedefs as much as possible and work
directly with the underlying types, to minimize namespace pollution
from signal.h in the default (_BSD_SOURCE) profile.

this is a first step in improving compatibility with applications
which poke at context/register information -- mainly debuggers, trace
utilities, etc. additional definitions in ucontext.h and other headers
may be needed later.

if feature test macros are used to request a conforming namespace,
mcontext_t is replaced with an opaque structure of the equivalent size
and alignment; conforming programs cannot examine its contents anyway.

these fields were wrongly copied from the kernel's ppc64 struct def

unlike the previous definition, NSIG/_NSIG is supposed to be one more
than the highest signal number. adding this will allow simplifying
libc-internal code that makes signal-related syscalls, which can be
done as a later step. some apps might use it too; while this usage is
questionable, it's at least not insane.

also handle the non-GNUC case where alignment attribute is not available
by simply omitting it. this will not cause problems except for
inclusion of mcontex_t/ucontext_t in application-defined structures,
since the natural alignment of the uc_mcontext member relative to the
start of ucontext_t is already correct. and shame on whoever designed
this for making it impossible to satisfy the ABI requirements without
GNUC extensions.

sigsetjmp: store temporaries in jmp_buf rather than on stack.

it's essential to decrement the stack pointer before writing to new
stack space, rather than afterwards. otherwise there is a race
condition during which asynchronous code (signals) could clobber the
data being stored.

it may be possible to optimize the code further using stwu, but I
wanted to avoid making any changes to the actual stack layout in this
commit. further improvements can be made separately if desired.

apparently some other archs have sys/io.h and should not break just
because they don't have the x86 port io functions. provide a blank
bits/io.h everywhere for now.

based on proposal by Isaac Dunham. nonexistance of bits/io.h will
cause inclusion of sys/io.h to produce an error on archs that are not
supposed to have it. this is probably the desired behavior, but the
error message may be a bit unusual.

based on code sent to the mailing list by nsz, with minor changes.

put some macros that do not differ between architectures in the
main header and remove from bits.
restructure mips header so it has the same structure as the others.

use the 'f' suffix when a float constant is not representable

raise overflow and underflow when necessary, fix various comments.

similar to exp.c cleanup: use scalbnf, don't return excess precision,
drop some optimizatoins.
exp.c was changed to be more consistent with expf.c code.