s390/uaccess: avoid mvcos jump label

If the kernel is compiled for z10 or later machines the uaccess
code inlines the mvcos instruction. The facility bit 27 which
indicates the availability of MVCOS has to be set. The have_mvcos
jump label will always be true.

Make the generation of the have_mvcos jump label conditional on
!CONFIG_HAVE_MARCH_Z10_FEATURES.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>