bootlader/grub2: disable x86-64-efi choice on !BR2_ARCH_IS_64

When x86-64-efi platform is selected, grub2 automatically adds -m64 to the
CFLAGS. This makes the configure script failed when the toolchain does not
have multilib support (like the Buildroot ones).

Reported-by: Noe Rubinstein <nrubinstein@aldebaran-robotics.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>