[PPC] Claim stack frame before storing into it, if no red zone is
present

Unlike PPC64, PPC32/SVRV4 does not have red zone. In the absence of
it there is no guarantee that this part of the stack will not be
modified by any interrupt. To avoid this, make sure to claim the
stack frame first before storing into it.

This fixes https://llvm.org/bugs/show_bug.cgi?id=26519.

Differential Revision: https://reviews.llvm.org/D24093