shim regressed cross building from source for multiple reasons. The
immediate failure is rooted in debian/rules passing CC=gcc to the
Makefile and thereby overriding the cross compiler choice via the
CROSS_COMPILE variable. Seeding it from buildtools.mk is a natural way
to fix this.

Nextup, -lelf is linked into buildid, which is a build architecture
tool. Therefore, the libelf-dev dependency needs to be annotated
:native.

Last but not least, debian/check_nx uses objdump. That tools is
architecture-dependent and needs to be prefixed with the host
architecture.

Once addressing all of these, shim cross builds again.

Helmut