Also apply WITH_LLD_AS_LD to build tools

Previously WITH_LLD_AS_LD installed LLD as /usr/bin/ld in the target
system, but still used the GNU BFD ld to link the binaries in that
target. LLD 4.0.0 can link the FreeBSD/amd64 world and kernel so use
LLD as the build-time linker as well when the knob is set.

Reviewed by: dim
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D9226