Install elftoolchain elfcopy(1) as objcopy(1) by default

As of r295661 elfcopy supports PE format for EFI boot binaries and is a
viable objcopy implementation for the base system and ports.

The (temporary) src.conf knob WITHOUT_ELFCOPY_AS_OBJCOPY knob may be set
to obtain the GNU version if necessary.

PR: 207091 [exp-run]
Relnotes: Yes
Sponsored by: The FreeBSD Foundation