[libc++abi][LIT][AIX] Use Vector instructions available on Power7 in vec_reg_restore.pass.cpp

Summary:
libc++abi LIT test case vec_reg_restore.pass.cpp for AIX uses instructions mtvsrd and mfvsrd that are only available on Power8 CPU and higher, and therefore, fails on Power7 which is supported by the current AIX Clang. This patch replaces mtvsrd/mfvsrd with vector instructions available on Power7.

Reviewed by: nemanjai

Differential Revision: https://reviews.llvm.org/D138667