[AIX][test/Index] Set/propagate AIXTHREAD_STK for AIX

Summary:
Some tests perform deep recursion, which requires a larger pthread stack
size than the relatively low default of 192 KiB for 64-bit processes on
AIX. The `AIXTHREAD_STK` environment variable provides a non-intrusive
way to request a larger pthread stack size for the tests. The required
pthread stack size depends on the build configuration.

A 4 MiB default is generous compared to the 512 KiB of macOS; however,
it is known that some compilers on AIX produce code that uses
comparatively more stack space.

Reviewers: xingxue, daltenty, jasonliu

Reviewed By: daltenty

Subscribers: arphaman, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65688

llvm-svn: 368690