cleanup of illumos compatibility atomics

atomic_cas_32 is implemented using atomic_fcmpset_32 on all platforms.
Ditto for atomic_cas_64 and atomic_fcmpset_64 on platforms that have it.
The only exception is sparc64 that provides MD atomic_cas_32 and
atomic_cas_64.
This is slightly inefficient as fcmpset reports whether the operation
updated the target and that information is not needed for cas.
Nevertheless, there is less code to maintain and to add for new platforms.
Also, the operations are done inline now as opposed to function calls before.

atomic_add_64_nv is implemented using atomic_fetchadd_64 on platforms
that provide it.

casptr, cas32, atomic_or_8, atomic_or_8_nv are completely removed as they
have no users.

atomic_mtx that is used to emulate 64-bit atomics on platforms that lack
them is defined only on those platforms.

As a result, platform specific opensolaris_atomic.S files have lost most of
their code. The only exception is i386 where the compat+contrib code
provides 64-bit atomics for userland use. That code assumes availability of
cmpxchg8b instruction. FreeBSD does not have that assumption for i386
userland and does not provide 64-bit atomics. Hopefully, this can and will
be fixed.

MFC after: 3 weeks