s390: Allow to compile with z17 optimizations

s390: Allow to compile with z17 optimizations

Add config and compile options which allow to compile with z17
optimizations if the compiler supports it. Add the
miscellaneous-instruction-extension 4 facility to the list of facilities
for z17.

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