cmd/ld: pass -Qunused-arguments to clang during host linking.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/82140043

src/cmd/ld/lib.c

@@ -562,7 +562,7 @@ hostlink(void)
 		p = strchr(p + 1, ' ');
 	}
 
-	argv = malloc((13+nhostobj+nldflag+c)*sizeof argv[0]);
+	argv = malloc((14+nhostobj+nldflag+c)*sizeof argv[0]);
 	argc = 0;
 	if(extld == nil)
 		extld = "gcc";
@@ -605,6 +605,9 @@ hostlink(void)
 	if(iself)
 		argv[argc++] = "-rdynamic";
 
+	if(strstr(argv[0], "clang") != nil)
+		argv[argc++] = "-Qunused-arguments";
+
 	// already wrote main object file
 	// copy host objects to temporary directory
 	for(i=0; i<nhostobj; i++) {