[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246510 91177308-0d34-0410-b5e6-96231b3b80d8