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 upstream
GH#3241.
(Imported from upstream's c47aea8af1e28e46e1ad5e2e7468b49fec3f4f29.)
This does not affect of the configurations we generate and is imported
to avoid a diff against upstream.
Change-Id: Iacde0ca5220c3607681fad081fbe72d8d613518f
Reviewed-on: https://boringssl-review.googlesource.com/15985
Reviewed-by: Adam Langley <agl@google.com>