Allow using sanitizers for ssp tests with out-of-tree compiler

With an out-of-tree Clang, we can use the -resource-dir flag when linking
to point it at the runtime libraries from the current SYSROOT.
This moves the path to the clang-internal library directory to a separate
.mk file that can be used by Makefiles that want to find the sanitizer
libraries. I intend to re-use this .mk file for my upcoming changes that
allow building the entire base system with ASAN/UBSAN/MSAN.

Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D28852