[AIX] Define _ARCH_PPC64 macro for 32-bit

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The macro _ARCH_PPC64 is already defined for 64-bit, but this patch defines it for 32-bit on AIX to follow xlc. See: https://www.ibm.com/docs/en/xl-c-and-cpp-aix/13.1.0?topic=features-macros-related-architecture-settings

Note: This change creates a discrepancy between GCC, which defines _ARCH_PPC64 only for 64-bit mode.

Tested with SPEC.
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Reviewed By: cebowleratibm

Differential Revision: https://reviews.llvm.org/D107244