linux-imx

um: Don't use generic barrier.h

UML has its own platform-specific barrier.h under arch/x86/um/,
which should get used. Fix the build system to use it, and then
fix the barrier.h to actually compile.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>