linux-imx

um: Run host archheaders, allow use of host generated headers

Run the "archheaders" target for the host architecture, for
architectures (like x86, now) that want to generate some of the
necessary header files.

Add $(HOST_DIR)/include/generated to the include path so we then pick
them up.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>