x86: efi: payload: Enforce toolchain to generate 64-bit EFI payload stub codes

Attempting to use a toolchain that is preconfigured to generate code
for the 32-bit architecture (i386), for example, the i386-linux-gcc
toolchain on kernel.org, to compile the 64-bit EFI payload does not
build. This updates the makefile fragments to ensure '-m64' is passed
to toolchain when building the 64-bit EFI payload stub codes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>