sh: oprofile: Use perf-events oprofile backend

Now that we've got a generic perf-events based oprofile backend we might
as well make use of it seeing as SH doesn't do anything special with its
oprofile backend. Also introduce a new CONFIG_HW_PERF_EVENTS symbol so
that we can fallback to using the timer interrupt for oprofile if the
CPU doesn't support perf events.

Also, to avoid a section mismatch warning we need to annotate
oprofile_arch_exit() with an __exit marker.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Robert Richter <robert.richter@amd.com>