perlasm/ppc-xlate.pl: Fix build on OS X

perlasm/ppc-xlate.pl: Fix build on OS X

vsr2vr1() fails on OS X because the main loop doesn't strip the
non-numeric register prefixes for OS X.

Strip any non-numeric prefix (likely just "v") from registers before
doing numeric calculation, then put the prefix back on the result.

Fixes: #16995
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17026)