Make asm/word-at-a-time.h available on all architectures

Added the x86 implementation of word-at-a-time to the
generic version, which previously only supported big-endian.

Omitted the x86-specific load_unaligned_zeropad(), which in
any case is also not present for the existing BE-only
implementation of a word-at-a-time, and is only used under
CONFIG_DCACHE_WORD_ACCESS.

Added as a "generic-y" to the Kbuilds of all architectures
that didn't previously have it.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

arch/mn10300/include/asm/Kbuild

@@ -8,3 +8,4 @@ generic-y += mcs_spinlock.h
 generic-y += preempt.h
 generic-y += sections.h
 generic-y += trace_clock.h
+generic-y += word-at-a-time.h