When single-stepping kernel code from xmon without a debug hook
enabled the kernel crashes. This can happen when kernel starts with
xmon on crash disabled but xmon is entered using sysrq.

Call force_enable_xmon when single-stepping in xmon to install the
xmon debug hooks.

Fixes: e1368d0c ("powerpc/xmon: Setup debugger hooks when first break-point is set")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>