Build Clang on all architectures if host compiler is C++11

Recent versions of Clang are becoming usable for non-x86 non-arm FreeBSD
architectures. If the compiler is capable of building Clang, do so in
order to facilitate further testing.

Reviewed by: dim, imp
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8267