Provide some 64-bit compiler intrinsics when compiling 32-bit on a 64- bit architecture

The xlC compiler supplies (many) intrinsics that be used in lieu of
assembly. Some of these intrinsic work on 32-bit words, and some on 64- bit double-words. (There are many others, but our interest here is 32- bit and 64-bit).
__lwarx() load-word-and-reserve
__stwcx() store-word-conditional
__ldarx() load-double-word-and-reserve
__stdcx() store-double-word-conditional

The latter two (operating on double-word) are only available when
compiling 64-bit, even if the architecture is running on a 64-bit POWER
CPU.

Similarly, xlC does not provide intrinsics for 64-bit compare-and-swap
when compiling in 32-bit. Need to compile 64-bit for
__compare_and_swaplp(). <sys/atomic.h> and AIX libc.a provide
compare_and_swaplp(), but the prototype is 'long' with is 32-bits in a
32-bit compile.

We request that xlC provide for *32-bit compilation* some 64-bit
intrinsics (when compiler target is 64-bit hardware architecture) that
operate on int64_t or uint64_t (uint64_t preferred), and/or provide
callable functions in 32-bit libc.a that operate on 64-bit quantities.

In 32-bit compilation, our other vendors provide simple solutions for
basic 64-bit compare-and-swap. We can provide details, if you are
interested.