aix: Fix broken cmpxchgi() XL C++ specialization.

The preprocessor was used to select a special implementation when
building on AIX using XL C++ (strangely, not XL C).  This code
implemented `cmpxchgi()` by directly reading the old value and then
calling `__compare_and_swap()`, an intrinsic that does not provide
any sort of memory barrier guarantees.  The return value was not
used, and the value read prior to the `__compare_and_swap()` call
was returned.  There is no way that this code could provide the
required semantics of the function and it causes observable data
races and strange library failures in production under load.

XL C/C++ for AIX has provided support for the GCC intrinsic used in
the GCC/Clang cases since version 12.1 of the compiler.  This
version of the compiler is old enough that it doesn't warrant a
version check.  (The compiler was released 8-Jun-2012, maintenance
ended 9-Jul-2019, and service ends 30-Apr-2020.)

This change fixes all observed atomic issues and unifies XL C/C++
with GCC/Clang.

Relevant XL C/C++ for AIX V12.1 documentation links:
  - [__compare_and_swap](https://www.ibm.com/support/knowledgecenter/en/SSGH3R_12.1.0/com.ibm.xlcpp121.aix.doc/compiler_ref/bif_compare_and_swap_compare_and_swaplp.html)
  - [__sync_val_compare_and_swap](https://www.ibm.com/support/knowledgecenter/en/SSGH3R_12.1.0/com.ibm.xlcpp121.aix.doc/compiler_ref/bif_gcc_atomic_val_comp_swap.html)

PR-URL: https://github.com/libuv/libuv/pull/2455
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

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