MIPS: Use compat_sys_ptrace

This replaces mips's sys_ptrace32 with a compat_arch_ptrace and
enables the new generic definition of compat_sys_ptrace instead.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>