Revise MIPS 64-bit ptrace interface

    
Change the N32 debugging ABI to something more sane, and add support
for o32 and n32 debuggers to trace n64 programs.
    
Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>