Remove PowerPC specific time reading function

Remove PowerPC specific time reading function

The PPC version of the trace_clock_read64 was using the TB register
which is not constant accross implementations. The currently
measured time base on PPC was not accurate.
So, for now, we rely on the CLOCK_MONOTONIC.

This patch remove a bunch of #ifdef for x86 and PPC, since we now rely on
the same base clock. It also fix the build on PPC that was currently broken

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>