Add OProfile support for metag, using the perf backend, and falling back
to generic timer based sampling if perf counter interrupt support is
disabled.

The oprofile code prepends "metag/" to the perf pmu name to give
"metag/meta2" which is more consistent with other oprofile arch names.

The backtrace code makes use of <asm/stacktrace.h> for kernel
backtracing, and a simple frame pointer walk for userland backtracing.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Robert Richter <rric@kernel.org>
Cc: oprofile-list@lists.sf.net