Apply ASAN/UBSAN Makefile check only for build target

COMPILER_TYPE is not set during cleandir and perhaps other non-build
targets, and a build with ASAN or UBSAN enabled failed with an error
reporting that runtime libraries could not be built.

PR: 260099
Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32805