[PowerPC] Enable -fomit-frame-pointer by default for PPC

As is the case on platforms like Mips, X86 and SystemZ, the -fomit-frame-pointer
should be enabled by default on PPC when optimizing at -O1 and above. This
brings the behaviour of LLVM on PPC in line with GCC.

Committing on behalf of Hiroshi Inoue.

Differential Revision: https://reviews.llvm.org/D29750

llvm-svn: 296861