powerpc/processor: Move cpu_relax() into asm/vdso/processor.h

cpu_relax() need to be in asm/vdso/processor.h to be used by
the C VDSO generic library.

Move it there.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201126131006.2431205-2-mpe@ellerman.id.au