arch, hweight: Fix compilation errors

Fix function prototype visibility issues when compiling for non-x86
architectures. Tested with crosstool
(ftp://ftp.kernel.org/pub/tools/crosstool/) with alpha, ia64 and sparc
targets.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
LKML-Reference: <20100503130736.GD26107@aftab>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>