arm: fix unaligned memcpy() and memmove()

arm: fix unaligned memcpy() and memmove()

If memcpy() and memmove() were used with source and destination of
different alignment then the result would be all jumbled.

When their implementations were imported from Linux some macros for
big-endian platforms were taken instead of the correct little-endian
ones (specifically, the push and pull macros in assembler.h).

Fix this by taking Linux's arch/include/asm/assembler.h as-is and
making only the minimum changes necessary for Xen.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <Ian.Campbell@citrix.com>