clang: allow -fstack-clash-protection on FreeBSD

-fstack-clash-protection was added in Clang commit e67cbac81211 but was
enabled only on Linux. It should work fine on FreeBSD as well, so
enable it.

To be discussed and upstreamed with a test. The OS test should probably
just be removed.

Reviewed by: dim
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27366