packages: Fix clang conditional

Various src.conf options can cause us to build something that ends up
in the clang package, but MK_TOOLCHAIN is not one of them; copy the
proper conditional from lib/Makefile to decide if we need to build
the package.

This fixes the build when LLVM/clang is entirely disabled.

Fixes: bb75b0d581f7 ("packages: Convert world to a subdir build")
MFC after: 2 weeks
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56657