Add WITH_LLD_BOOTSTRAP and WITHOUT_LLD_BOOTSTRAP knobs similar to the Clang bootstrap ones.
The three knobs are:
WITH_LLD -> Builds /usr/bin/ld.lld
WITH_LLD_BOOTSTRAP -> lld is used as the buildworld linker
WITH_LLD_IS_LD -> The installed /usr/bin/ld is a symlink to ld.lld
This is a work in progress, initially posted for confirmation that these knobs are the desired approach.