[AIX][test/Index] Set/propagate AIXTHREAD_STK for AIX
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
Differential Revision: https://reviews.llvm.org/D65688