sparc64: Clean up handling of pt_regs trap type encoding.

If we use this from more than one place, it's better to
have helpers instead of twiddling magic constants all
over.

Add pt_regs_trap_type(), pt_regs_clear_trap_type(), and
pt_regs_is_syscall().

Use them in do_signal().

Signed-off-by: David S. Miller <davem@davemloft.net>