Reapply r158337, this time properly protect Darwin/PPC host CPU use with __ppc__.

Original commit message:
Move PPC host-CPU detection logic from PPCSubtarget into sys::getHostCPUName().

Both the new Linux functionality and the old Darwin functions have been moved.
This change also allows this information to be queried directly by clang and
other frontends (clang, for example, will now have real -mcpu=native support).

llvm-svn: 158349