aes/asm/aesp8-ppc.pl: improve [backward] portability.

Some of stone-age assembler can't cope with r0 in address. It's actually
sensible thing to do, because r0 is shunted to 0 in address arithmetic
and by refusing r0 assembler effectively makes you understand that.

Reviewed-by: Rich Salz <rsalz@openssl.org>