Revert "rt-tests: Enable only for x86/ppc64 architectures"

This reverts commit 44010756.

With the packported patch from rt-tests (67da9d8af7d8a0e1a0822e6ee99d68fa3d5a46d2)
that allows build for all archs this patch can be reverted. An error is dumped in
run-time is frc() is not present.

Signed-off-by: Peter Bergin <peter@berginkonsult.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>