MIPS/Linux: Also recognize TRAP_BRKPT and TRAP_HWBKPT

This makes the MIPS Linux backends recognize TRAP_BRKPT and
TRAP_HWBKPT in siginfo.si_code in addition to SI_KERNEL, since Linux
4.6 now reports the finer-grained si_code values too.

Refs:
 https://sourceware.org/ml/gdb-patches/2016-02/msg00756.html
 https://sourceware.org/ml/gdb-patches/2016-04/msg00090.html

On kernels that report SI_KERNEL (<= 4.5), we'll enter the "ambiguous"
path of save_stop_reason:

	  if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code)
	      && GDB_ARCH_IS_TRAP_HWBKPT (siginfo.si_code))
	    {
	      /* The si_code is ambiguous on this arch -- check debug
		 registers.  */
	      if (!check_stopped_by_watchpoint (lp))
		lp->stop_reason = TARGET_STOPPED_BY_SW_BREAKPOINT;
	    }

while on kernels that report the finer-grained si_code values (>= 4.6),
we'll enter the corresponding branches:

	  else if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code))
	    {
	    }
	  else if (GDB_ARCH_IS_TRAP_HWBKPT (siginfo.si_code))
	    {
	      ...

gdb/ChangeLog:
2016-04-15  Pedro Alves  <palves@redhat.com>

	* nat/linux-ptrace.h [__mips__] (GDB_ARCH_IS_TRAP_BRKPT): Also
	accept TRAP_BRKPT.
	 [__mips__] (GDB_ARCH_IS_TRAP_HWBKPT): Also accept TRAP_HWBKPT.