llvm: remove __FreeBSD_version conditionals

All supported FreeBSD build host versions have backtrace.h, so we can
just eliminate that test. For futimes() we can test the compiler's
built-in FreeBSD major version rather than relying on including
osreldate.h. This should reduce the frequency with which Clang gets
rebuilt when building world.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation