sandbox: allow cross-compiling sandbox

UEFI test files like helloworld.efi require an architecture specific
PE-COFF header.

Currently this does not work for cross compiling. If $CROSS_COMPILE is set,
use the first part of the architecture triplet from the variable to
choose the PE-COFF header.

Now we can cross-compile the sandbox, e.g.

    make sandbox_defconfig NO_SDL=1
    CROSS_COMPILE=/opt/bin/aarch64-linux-gnu- NO_SDL=1 MK_ARCH=aarch64 make

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>