poly1305-ppc.pl: Fix vector register clobbering

Fixes CVE-2023-6129

The POLY1305 MAC (message authentication code) implementation in OpenSSL for
PowerPC CPUs saves the the contents of vector registers in different order
than they are restored. Thus the contents of some of these vector registers
is corrupted when returning to the caller. The vulnerable code is used only
on newer PowerPC processors supporting the PowerISA 2.07 instructions.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23200)