[PPC64, Sanitizers] Proper stack frame for the thread spawned in internal_clone

Summary:
Set up the proper stack frame for the thread spawned in internal_clone,
the current code does not follow ABI (and causes SEGV trying to use this
malformed frame).

Reviewers: wschmidt

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31555

llvm-svn: 299896