efi_loader: always rebuild efi_crt0.o

When changing the architecture without calling 'make clean' a subsequent
make fails with
lib/efi_loader/efi_crt0.o: file not recognized: File format not recognized

Force efi_crt0.o to be always rebuild.

Reported-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>