[libcxx][AIX] Enable ABI list checking for XCOFF

The existing nm extractors can't dump the loader symbol table information we need to do the ABI checks for XCOFF, so provide an implementation using the system dump utility. We match the symbol name, whether it's defined, it's import/export status, and its storage mapping class.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D124165