powerpc/perf: Move BHRB code into CONFIG_PPC64 region

The new Branch History Rolling buffer (BHRB) code is only useful on 64bit
processors, so move it into the #ifdef CONFIG_PPC64 region.

This avoids code bloat on 32bit systems.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>