Use PIC relocation mode by default for PowerPC64 ELF

Most of the PowerPC64 code generation already creates PIC access. This
changes to a full PIC default, similar to what GCC is doing.

Overall, a monolithic clang binary shrinks by 600KB (about 1%). This can
be a slight regression for TLS access and will use the TOC more
aggressively instead of synthesizing immediates. It is expected to be
performance neutral.

Differential Revision: https://reviews.llvm.org/D26564

llvm-svn: 289744