Switch to ld.bfd as default ld for world stage.

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.