ARM assembly adjustments for big-endian systems contained armv6+-only
instructions (rev) in generic arm memxor code. Replace those with an
actual conversion of the leftover byte store routines for big-endian
systems. This also provides a slight optimisation by removing the
additional instruction as well as increased symmetry between little- and
big-endian implementations.
Signed-off-by: Michael Weiser <michael.weiser@gmx.de>