Only use ASAN when using the in-tree compiler

When building FreeBSD 11 on a FreeBSD 12 system with
CROSS_TOOLCHAIN=llvm10 we end up trying to link against the packaged
version of the sanitizer library. This resulted in a requirement for
getentropy(3) which is not present in FreeBSD 11.

Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D26903