On powerpc, probing on emulate_step function will crash 2.6.18.1 when
it is triggered.
When kprobe is triggered, emulate_step() is on its kernel path and
will cause recursive kprobe fault. And branch_taken() is called
in emulate_step(). This disallows kprobes on both of them.
Signed-off-by: Paul Mackerras <paulus@samba.org>