docs/cross-building.txt: add documentation about cross-builds

Cross-building IGT is not too trivial, as one needs to prepare
a sysroot directory containing not only glibc but also other
libraries. Document how to do it for IGT.

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>