[PPC] Fix 32-bit build of libunwind

Clang integrated assembler was unable to build libunwind PPC32 assembly code,
present in functions used to save/restore register context.

This change consists in replacing the assembly style used in libunwind source,
to one that is compatible with both Clang integrated assembler as well as
GNU assembler.

Patch by Leandro Lupori!

Differential Revision: https://reviews.llvm.org/D61792

llvm-svn: 360862