build: fix llvm version detection in freebsd-10

In FreeBSD-10, the banner of clang version is "FreeBSD clang version".
Fix regex to detect it.

PR-URL: https://github.com/nodejs/node/pull/11668


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>