musl lacks __ppc_get_timebase() but has __builtin_ppc_get_timebase()
the __ppc_get_timebase_freq() is taken from:
https://git.alpinelinux.org/aports/commit/?id=06b03f70fb94972286c0c9f6278df89e53903833
Signed-off-by: Duncan Bellamy <dunk@denkimushi.com>