Extend the OS X pthread_get_stacksize_np workaround to 10.10

Fixes a failing ASan testcase (TestCases/stack-use-after-return.cc) on OS X 10.10.

Reviewed at http://reviews.llvm.org/D6120



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@221380 91177308-0d34-0410-b5e6-96231b3b80d8