check-bin-arch: skip /lib/modules to allow 32-bit userland on 64-bit arch

The script check-bin-arch fails as follows on a config for PowerPC e6500
(64-bit CPU) with BR2_ARCH="powerpc" (32-bit userland desired):

ERROR: architecture for "/lib/modules/..../lib/libcrc32c.ko"
            is "PowerPC64", should be "PowerPC"

This situation is perfectly acceptable: the kernel is 64-bit and so are its
modules, even though userland is 32-bit.

To keep check-bin-arch and its caller simple, just skip /lib/modules/
entirely, like is done for /lib/firmware and some others.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>