[ARM] 3252/1: help gcc do the best with ___arch__swab32

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: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>