[AIX][CMake] Changes for building on AIX with XL and GCC

Summary:
In support of IBM's efforts to produce a viable C and C++ LLVM compiler for AIX
(ref: RFC at http://lists.llvm.org/pipermail/llvm-dev/2019-February/130175.html),
this patch adds customizations to the CMake files in order to properly
invoke the host toolchain for the build on AIX.
Additional changes to enable a successful build will follow.

Patch by Xing Xue

Reviewers: hubert.reinterpretcast, jasonliu, sfertile

Reviewed by: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D58250

llvm-svn: 356104