powerpc/Makefile: Add ppc32/ppc64_randconfig targets

Make it easier to generate a 32 or 64-bit specific randconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Requested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210428132700.3426100-1-mpe@ellerman.id.au