op_name_from_perf_id() currently returns a local variable, which isn't
terribly productive. As we only handle a single PMU case for now, simply
allocate and free the string from the arch init/exit context and have
op_name_from_perf_id() hand back the cached string.

This also takes UTS_MACHINE in to account, given that we build for
multiple architectures.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>