openrisc: don't use pt_regs in struct sigcontext

As it was decided not to export struct pt_regs to userspace, struct
sigcontext shouldn't be using it either.  The pt_regs struct for OpenRISC
is kernel internal and the layout of the registers may change in the
future.  The struct user_regs_struct is what is guaranteed to remain
stable, so struct sigcontext may use that instead.

This patch removes the usage of struct pt_regs in struct sigcontext and
makes according changes in signal.c to get the register layout right.

The usp field is removed from the sigcontext structure as this information
is already contained in the user_regs_struct.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Reviewed-by: Emilio Cota <cota@braap.org>