boot: Always use ELF Tool Chain elfcopy for EFI builds

We now use llvm-objcopy by default (as of commit 1cae7121c667), but it
does not support efi-app-x86_64 and similar ouptut formats (for more
detail see LLVM issue 108609[1]).

Go back to installing ELF Tool Chain's version of objcopy as elfcopy
(the standard upstream name) and use it for EFI builds.

[1] https://github.com/llvm/llvm-project/issues/108609).

PR: 280771
Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49362