Add AIX Target Info

Summary:
A first pass over platform-specific properties of the C API/ABI
on AIX for both 32-bit and 64-bit modes.
This is a continuation of D18360 by Andrew Paprocki and further work by Wu Zhao.

Patch by Andus Yu

Reviewers: apaprocki, chandlerc, hubert.reinterpretcast, jasonliu,
xingxue, sfertile

Reviewed by: hubert.reinterpretcast, apaprocki, sfertile

Differential Revision: https://reviews.llvm.org/D59048

llvm-svn: 356060