OpenSSL: Fix handling of the "0:" label in arm-xlate.pl

When $label == "0", $label is not truthy, so if ($label) thinks there isn't
a label. Correct this by looking at the result of the s/// command.

Verified that there are no changes in the .S files created during a normal
build, and that the "0:" labels appear in the translation given in the error
report (and they are the only difference in the before and after output).

Obtained from: OpenSSL commit 9607f5ccf285ac9988a86f95c5ad9f92b556a843

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41587