sh: Fix irqflags tracing for SH-3/4 nommu.

We were missing the trace_hardirqs_on() instrumentation in the nommu
case, resync with the MMU version of the page fault handler to have
this behaving consistently. Also explicitly re-enable IRQs now that
the assembly code isn't doing it for us any more.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>