[libsanitizer] Workaround for https://code.google.com/p/address-sanitizer/issues/detail?id=261
If pthread_get_stacksize_np() returns 512K for the main thread on Mavericks, obtain the stack size from the current stack rlimit.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200703 91177308-0d34-0410-b5e6-96231b3b80d8