Rework how the ld link is handled in WORLDTMP from r322811.

LLD_BOOTSTRAP (build) is independent of LLD_IS_LD (installed) so they
should not be based on each other.

This is related to upcoming WITH_SYSTEM_LINKER work.

Reviewed by: emaste
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D15836