lang/gnat1?: fix build with GNU strip

After D55509 $STRIP_CMD has been set to GNU strip for these ports, and in some
cases this breaks, because GNU strip will refuse to strip non-writable files,
while LLVM strip is happy to (attempt to) strip them.

PR: 294657
Reported by: kib@FreeBSD.org
Reviewed by: kib@FreeBSD.org
Phabricator: D55509
MFC after: 8 days