Define llvm::thread::DefaultStackSize to 4 megabytes on AIX

Link time thinLTO spawns pthreads to parallelize optimization and
codegen of the input bitcode files. On AIX, the default pthread
stack size limit is ~192k for 64-bit programs; insufficient for a
normal LLVM compilation.

Reviewed By: ZarkoCA, MaskRay

Differential Revision: https://reviews.llvm.org/D155731