[PowerPC][AIX] Warn when using pragma align(packed) on AIX.

With xlc and xlC pragma align(packed) will pack bitfields the same way
as pragma align(bit_packed). xlclang, xlclang++ and clang will
pack bitfields the same way as pragma pack(1). Issue a warning when
source code using pragma align(packed) is used to alert the user it
may not be compatable with xlc/xlC.

Differential Revision: https://reviews.llvm.org/D107506