barebox: fix compilation on x86-64 architecture

Barebox uses KERNEL_ARCH to derive BAREBOX_ARCH, but doesn't
currently handle the case of the x86-64 architecture. In this
case KERNEL_ARCH is x86_64, but BAREBOX_ARCH should be x86
nonetheless.

Signed-off-by: Fabien Lahoudere <fabienlahoudere.pro@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>