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.

sys/mips/mips/trap.c

@@ -840,6 +840,7 @@ dofault:
 			if (td->td_md.md_ss_addr != va ||
 			    instr != MIPS_BREAK_SSTEP) {
 				i = SIGTRAP;
+				ucode = TRAP_BRKPT;
 				addr = trapframe->pc;
 				break;
 			}
@@ -851,6 +852,7 @@ dofault:
 			 */
 			addr = trapframe->pc;
 			i = SIGTRAP;
+			ucode = TRAP_TRACE;
 			break;
 		}
 
@@ -865,6 +867,7 @@ dofault:
 				va += sizeof(int);
 			printf("watch exception @ %p\n", (void *)va);
 			i = SIGTRAP;
+			ucode = TRAP_BRKPT;
 			addr = va;
 			break;
 		}