Switch to ld.bfd as default ld for world stage.
Until boot/efi issues are resolved:
* ld.gold not respecting linker script for section merges;
* clang built boot1.efi and loader.efi failures.
Default can be restored by setting WORLD_LDVER=ld.gold in make.conf(5).
Final installed ld(1) will still default to ld.gold to not disturb
things in DPorts.
NOTE: this might produce different final binaries if building in tree,
but it should not be fatal as we support both ld.bfd and ld.gold.
Use "make LDVER=ld.bfd" to mimic buildworld behaviour when needed.