implement line number info tracking

Support "file" and "loc" directives. "file" takes a string (a file name)
assigns it a number, sets the current file to that number and records
the string for later. "loc" takes a single number and outputs location
information with a reference to the current file.

amd64/emit.c

@@ -547,6 +547,9 @@ emitins(Ins i, Fn *fn, FILE *f)
 		emitcopy(i.arg[0], i.arg[1], i.cls, fn, f);
 		emitcopy(i.arg[1], TMP(XMM0+15), i.cls, fn, f);
 		break;
+	case Oloc:
+		emitdbgloc(i.arg[0].val, f);
+		break;
 	}
 }
 

amd64/isel.c

@@ -392,6 +392,7 @@ sel(Ins i, ANum *an, Fn *fn)
 	case_Oload:
 		seladdr(&i.arg[0], an, fn);
 		goto Emit;
+	case Oloc:
 	case Ocall:
 	case Osalloc:
 	case Ocopy: