Don't override the user-provided ARCH when cross-compiling otherwise
ARCH won't be correct for armv5, aarch64 and x86_64
Fixes:
- http://autobuild.buildroot.org/results/2dfc0e10da25a8382a43557420d7dc3444c02dbb
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
