Use AIX version detection at LLVM run-time

Summary:
AIX compilers define macros based on the version of the operating
system.

This patch implements updating of versionless AIX triples to include the
host AIX version. Also, the host triple detection in the build system is
adjusted to strip the AIX version information so that the run-time
detection is preferred.

Reviewers: xingxue, stefanp, nemanjai, jasonliu

Reviewed By: xingxue

Subscribers: mgorny, kristina, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58798

llvm-svn: 355995