the a_cas_l, a_swap_l, a_swap_p, and a_store_l operations were
probably used a long time ago when only i386 and x86_64 were
supported. as other archs were added, support for them was
inconsistent, and they are obviously not in use at present. having
them around potentially confuses readers working on new ports, and the
type-punning hacks and inconsistent use of types in their definitions
is not a style I wish to perpetuate in the source tree, so removing
them seems appropriate.

previously I had wrongly assumed the ll/sc instructions also provided
memory synchronization; apparently they do not. this commit adds sync
instructions before and after each atomic operation and changes the
atomic store to simply use sync before and after a plain store, rather
than a useless compare-and-swap.

despite lacking the semantic content that the asm accesses the
pointed-to object rather than just using its address as a value, the
mips asm was not actually broken. the asm blocks were declared
volatile, meaning that the compiler must treat them as having unknown
side effects.

however changing the asm to use memory constraints is desirable not
just from a semantic correctness and consistency standpoint, but also
produces better code. the compiler is able to use base/offset
addressing expressions for the atomic object's address rather than
having to load the address into a single register. this improves
access to global locks in static libc, and access to non-zero-offset
atomic fields in synchronization primitives, etc.

this is needed for recently committed sigaction code

linux guarantees ll/sc are always available. on mips1, they will be
emulated by the kernel. thus they are part of the linux mips1 abi and
safe to use.

sc was overwriting the result

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.