rt-tests: Enable only for x86/ppc64 architectures

It needs frc() implementation for oslat, this is new tool added to
rt-tests recently, and there is no easy knob to configure it out at
present

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>