kbuild: soften modpost checks when doing cross builds

The module alias support in the kernel have a consistency
check where it is checked that the size of a structure
in the kernel and on the build host are the same.
For cross builds this check does not make sense so detect
when we do cross builds and silently skip the check in these
situations.
This fixes a build bug for a wireless driver when cross building
for arm.

Acked-by: Michael Buesch <mb@bu3sch.de>
Tested-by: Gordon Farquharson <gordonfarquharson@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: stable@kernel.org
diff --git a/Makefile b/Makefile
index 7a4d34b..1c4c8f8 100644
--- a/Makefile
+++ b/Makefile
@@ -189,7 +189,7 @@
 # Alternatively CROSS_COMPILE can be set in the environment.
 # Default value for CROSS_COMPILE is not to prefix executables
 # Note: Some architectures assign CROSS_COMPILE in their arch/*/Makefile
-
+export KBUILD_BUILDHOST := $(SUBARCH)
 ARCH		?= $(SUBARCH)
 CROSS_COMPILE	?=