[PowerPC] Support __builtin_ppc_get_timebase

GCC 4.8+ has a PowerPC-specific intrinsic, __builtin_ppc_get_timebase, to do
what Clang's __builtin_readcyclecounter does. For compatibility with code that
uses GCC's spelling (including glibc), support it as well.

Partially fixes PR23681.

llvm-svn: 246510