um, hweight: Fix UML boot crash due to x86 optimized hweight

Apparently UML cannot stomach callee reg-saving trickery
introduced with d61931d8
(x86: Add optimized popcnt variants) and oopses during boot:
http://marc.info/?l=linux-kernel&m=127522065202435&w=2



Redirect arch_hweight.h include from the x86 portion to the generic
arch_hweight.h which is a fallback to the software hweight routines.

LKML-Reference: <201005271944.09541.toralf.foerster@gmx.de>
Signed-off-by: Borislav Petkov <bp@alien8.de>
LKML-Reference: <4C0F4B00.4090307@panasas.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>