[AIX] Fix Link Issue when `-fprofile-update=[atomic|prefer-atomic]` is in Effect

https://reviews.llvm.org/D157280 enabled `-fprofile-update` for `-fprofile-generate`, but omitted adding `-latomic` to the linker command on AIX. This omission causes linking to fail due to an undefined symbol. This patch fixes the link error.

Reviewed By: w2yehia

Differential Revision: https://reviews.llvm.org/D159137