perlasm/x86_64-xlate.pl: work around problem with hex constants in masm.

Perl, multiple versions, for some reason occasionally takes issue with
letter b[?] in ox([0-9a-f]+) regex. As result some constants, such as
0xb1 came out wrong when generating code for MASM. Fixes GH#3241.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3385)