new helper: signal_pt_regs()

Always equal to task_pt_regs(current); defined only when we are in
signal delivery.  It may be different from current_pt_regs() - e.g.
architectures like m68k may have pt_regs location on exception
different from that on a syscall and signals (just as ptrace handling)
may happen on exceptions as well as on syscalls.

When they are equal, it's often better to have signal_pt_regs
defined (in asm/ptrace.h) as current_pt_regs - that tends to be
optimized better than default would be.  However, optimisation is
the only reason why we might want an arch-specific definition;
if current_pt_regs() and task_pt_regs(current) have different
values, the latter one is right.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>