s390/Kconfig: make z196 the default processor type

The current default processor type is z900. The BPF jit compiler
depends on PACK_STACK && HAVE_MARCH_Z196_FEATURES. To have the
BPF jit code included in compiles with 'make allmodconfig' set
the default processor type to z196.

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