boot: Fall back to objcopy if usable for EFI components

In commit b885643b63e4 ("boot: Always use ELF Tool Chain elfcopy for EFI
builds") I switched to using elfcopy for EFI_OBJCOPY, as llvm-objcopy
lacks support for translating ELF objects to PE32+. This broke building
on older releases with WITHOUT_ELFTOOLCHAIN_BOOTSTRAP set, as the host
did not provide elfcopy and it was not built as a bootstrap tool.

Now, if we're not bootstrapping ELF Tool Chain we check the output of
${OBJCOPY} --version, and will use that ${OBJCOPY} as long as it is
not llvm-objcopy.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49722