[ARM] Fix byte order macros for Thumb

The byte order functions are visible to userspace.  Unfortunately,
__arch_swab32() contains an assembly instruction which is invalid when
compiling for Thumb.  This reverts to the C version when compiling for
Thumb.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>