[PowerPC] Implement XL compatible behavior of __compare_and_swap

According to https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=functions-compare-swap-compare-swaplp
XL's `__compare_and_swap` has a weird behavior that

> In either case, the contents of the memory location specified by addr are copied into the memory location specified by old_val_addr.

(unlike c11 `atomic_compare_exchange` specified in http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf)

This patch let clang's implementation follow this behavior.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D106344