uprobes/powerpc: Add dependency on single step emulation

Uprobes uses emulate_step in sstep.c, but we haven't explicitly specified
the dependency. On pseries HAVE_HW_BREAKPOINT protects us, but 44x has no
such luxury.

Consolidate other users that depend on sstep and create a new config option.

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Suzuki K. Poulose <suzuki@in.ibm.com>
Cc: linuxppc-dev@ozlabs.org
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>