[AIX][compiler-rt] Pick the right form of COMPILER_RT_ALIAS for AIX

Summary: we use the alias attribute, similar to what is done for ELF.

Reviewers: ZarkoCA, jasonliu, hubert.reinterpretcast, sfertile

Reviewed By: jasonliu

Subscribers: dberris, aheejin, mstorsjo, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D81120