Report proper signal codes for SIGTRAP traps on MIPS.

• Use TRAP_TRACE for traps after stepping via PT_STEP.
• Use TRAP_BRKPT for software breakpoint traps and watchpoint traps.

This was tested via the recently added siginfo ptrace() tests. PT_STEP on
MIPS has several bugs that prevent it from working yet, but this does fix
the ptrace__breakpoint_siginfo test on MIPS.