uml: headers are not exportable

uml: headers are not exportable

When building UML this showed up as of late (2022-09-01):
| Makefile:1335: *** Headers not exportable for UML.  Stop.

This message popped up because of an upstream patch that
fixed this error not showing up sooner. For more information see:
<https://lore.kernel.org/lkml/20220901011252.550830-1-masahiroy@kernel.org/T/>

Now, I don't think we can just skip the kernel headers.
Some userspace applications are going to need these. So,
The next best alternative I can think of, is to go with
the headers by the host arch the UML is compiled for.
(Technically, ARCH= should be safe for all other targets
as well... But let's not poke a hornet's nest)

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>