Always build the sanitizer runtimes when compiling with clang

This allows instrumenting e.g. test binaries even when compiling with an
external clang (e.g. CROSS_TOOLCHAIN=llvm11). I have some upcoming patches
that allow building the entire base system with ASan/UBSan/etc.
instrumentation and this is required in preparation for this.

Reviewed By: dim, emaste
Differential Revision: https://reviews.freebsd.org/D28532