[AIX] Avoid use of mtim.tv_nsec member of stat structure on AIX

The value observed for the `mtim.tv_nsec` member is erroneous in some
AIX environments. Avoid using this member by forcing
`HAVE_STRUCT_STAT_ST_MTIM_TV_NSEC` to `0`.

This resolves "mtime changed" errors such as the one
http://lab.llvm.org:8014/#/builders/126/builds/330/steps/5/logs/FAIL__Clang__test_c
has.

Reviewed By: daltenty, jsji

Differential Revision: https://reviews.llvm.org/D101323