ld: parse but do not implement -X flag

This will let programs invoking ld prepare for it.
See issue 2676.

R=iant
CC=golang-dev
https://golang.org/cl/5535044

src/cmd/5l/obj.c

@@ -136,6 +136,11 @@ main(int argc, char *argv[])
 	case 'V':
 		print("%cl version %s\n", thechar, getgoversion());
 		errorexit();
+	case 'X':
+		// TODO: golang.org/issue/2676
+		EARGF(usage());
+		EARGF(usage());
+		break;
 	} ARGEND
 
 	USED(argc);

src/cmd/6l/obj.c

@@ -132,6 +132,11 @@ main(int argc, char *argv[])
 	case 'V':
 		print("%cl version %s\n", thechar, getgoversion());
 		errorexit();
+	case 'X':
+		// TODO: golang.org/issue/2676
+		EARGF(usage());
+		EARGF(usage());
+		break;
 	} ARGEND
 
 	if(argc != 1)

src/cmd/8l/obj.c

@@ -137,6 +137,11 @@ main(int argc, char *argv[])
 	case 'V':
 		print("%cl version %s\n", thechar, getgoversion());
 		errorexit();
+	case 'X':
+		// TODO: golang.org/issue/2676
+		EARGF(usage());
+		EARGF(usage());
+		break;
 	} ARGEND
 
 	if(argc != 1)