[SPARC64]: Store magic cookie and trap type in pt_regs.

This sets us up for several simplifications and facilities:

1) The magic cookie lets us identify trap frames more
   accurately in stack backtraces.

2) The trap type lets us simplify all of the "are we in
   a syscall" state management and checks.

3) We can now see if a task off the cpu is sleeping in
   a system call or not.  In fact, we can see what
   trap it is sleeping in whatever the type.  The utrace
   guys will use this.

Based upon some discussions with Roland McGrath.

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