[S390] Fix compile error in swab.h

The inline assembly in__arch_swab16p causes compile errors of the form:

*error*: *invalid* '*asm*': operand number missing after %-*letter*

The assembly uses the %O<n>/%R<n> notation but the first operand misses
the operand number, it needs to be "%O1" instead of "%O".

Reported-by: Gil Peleg <gilpeleg@servframe.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>