unix: Support atomic compare & swap xlC on AIX
unix: Support atomic compare & swap xlC on AIX

On AIX with the native xlC compiler, the compiler supports sync
compare swap and compare in two flavors:
 1 __compare_and_swap
 2 __compare_and_swaplp

This differs from the gcc version of this function in name. The
second version of this function supports 8-byte boundary for
a doubleword. Therefore, the macro logic checks:
 if (AIX OS AND using xlC compiler)
    if (64bit)
       __compare_and_swaplp /* 64 bit version */
    else
       __compare_and_swap
    endif
 endif

Signed-off-by: Neil Mushell <nmushell@bloomberg.net>

PR-URL: https://github.com/libuv/libuv/pull/521
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Michael Dawson <241bf52ebd97cc4ec83d57b04d1f9e9053df032e@ca.ibm.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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