Set MK_LLD_IS_LD to MK_LLD_BOOTSTRAP for cross-tools

LLD_BOOTSTRAP is intended to control the linker used to link world and
kernel, while LLD_IS_LD is intended to control the linker installed in
that world.

Force LLD_IS_LD equal to LLD_BOOTSTRAP for the cross-tools build and
install phase, so that lld will be installed as the ld to run on the
host, when LLD_BOOTSTRAP is set.

PR: 221543
Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12072