Handle "efi-app-riscv64" and similar targets in objcopy.

This adds the efi target name handling for riscv64 to objcopy.

binutils:
	* binutils/objcopy.c: add riscv64 handling to
	  convert_efi_target()

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>