s390/Kconfig: use builtin bswap
s390/Kconfig: use builtin bswap

Depending on the gcc version we can use builtin_bswap instead of
architecture functions. Doing so is better than the inline assembly
version of load reverse for two reasons:
- the sequence of load reversed, apply constant mask, save reversed can
  be optimized to load, apply reversed mask, save
- builtins are slightly better to optimize e.g. gcc instruction
  scheduler cannot optimize grouping on inline assemblies.

To enable set we have to ARCH_USE_BUILTIN_BSWAP.
bloat-o-meter results:
add/remove: 1/1 grow/shrink: 75/533 up/down: 1711/-9394 (-7683)

Suggested-by: David Woodhouse <b460d66aaf00c296a3db1c1d9eeafc081d5f7d70@intel.com>
Signed-off-by: Christian Borntraeger <49712f635b03c16d7f4c43df70e49ec8c565813d@de.ibm.com>
Acked-by: Heiko Carstens <8dcf0f69152f32f23184f83357a3731522e56b9c@de.ibm.com>
Signed-off-by: Martin Schwidefsky <52616596d8f5df0d597e85ab515377f92f939c68@de.ibm.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs