Pass pure constants verbatim in perlasm/x86_64-xlate.pl

(Imported from upstream's 10c639a8a56c90bec9e332c7ca76ef552b3952ac)

Change-Id: Ia8203eeae9d274249595a6e352ec2f77a97ca5d5
Reviewed-on: https://boringssl-review.googlesource.com/7227
Reviewed-by: David Benjamin <davidben@google.com>