s390: Always enable EXPOLINE_EXTERN if supported

s390: Always enable EXPOLINE_EXTERN if supported

Since commit ba05b39d54ee ("s390/expoline: Make modules use kernel
expolines"), there is no longer any reason not to use
CONFIG_EXPOLINE_EXTERN when supported by the compiler.

On the positive side:
- there is only a single set of expolines generated and used by both the
  kernel code and modules,
- it eliminates expolines "comdat" sections, which can confuse tools
  like kpatch.

Always enable EXPOLINE_EXTERN if supported by the compiler.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>