Patch from Nicolas Pitre

Depending on your gcc version, the current C-only implementation would
produce suboptimal code, ranging from a bad register selection forcing
an additional mov instruction to a failure to merge the eor and the ror
in a single instruction.  With a little help gcc always produces the
best code.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>