sh64: fix tracing of signals.

This follows the parisc change to ensure that tracehook_signal_handler()
is aware of when we are single-stepping in order to ptrace_notify()
appropriately. While this was implemented for 32-bit SH, sh64 neglected
to make use of TIF_SINGLESTEP when it was folded in with the 32-bit code,
resulting in ptrace_notify() never being called.

As sh64 uses all of the other abstractions already, this simply plugs in
the thread flag in the appropriate enable/disable paths and fixes up the
tracehook notification accordingly. With this in place, sh64 is brought
in line with what 32-bit is already doing.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>