KVM: selftests: Ensure sysreg-defs.h is generated at the expected path

Building the KVM selftests from the main selftests Makefile (as opposed
to the kvm subdirectory) doesn't work as OUTPUT is set, forcing the
generated header to spill into the selftests directory. Additionally,
relative paths do not work when building outside of the srctree, as the
canonical selftests path is replaced with 'kselftest' in the output.

Work around both of these issues by explicitly overriding OUTPUT on the
submake cmdline. Move the whole fragment below the point lib.mk gets
included such that $(abs_objdir) is available.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20231212070431.145544-2-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>