[POWERPC] Fix ppc32 single-stepping out of syscalls

The ppc32 kernel didn't properly set/clear the TIF_SINGLESTEP
flag, causing return from syscalls to not SIGTRAP, thus executing
one more instruction before stopping again.

This fixes it.  The ptrace code is a bit of a mess, and is overdue
for at least a -proper- 32/64 bits split and possibly more cleanups
but this minimum fix should be ok for 2.6.22

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>