Stop clang 13 from defining conflicting macros on PowerPC
Merge commit c9539f957f57 from llvm git (by Nemanja Ivanovic):
[PowerPC] Define XL-compatible macros only for AIX and Linux Since XLC only ever shipped on PowerPC AIX and Linux, it is not reasonable to provide the compatibility macros on any target other than those two. This patch restricts those macros to AIX/Linux. Differential revision: https://reviews.llvm.org/D110213
PR: 258209
(cherry picked from commit 39dadd0628cd1f28be39a3ab35969545b7401623)