cmd/ld: close outfile before cleanup

This prevents the temporary directory from being leaked when
the linker is run on a FUSE filesystem.

Fixes #8684.

LGTM=bradfitz
R=golang-codereviews, rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/141840043

src/cmd/ld/lib.c

@@ -531,8 +531,9 @@ char* mktempdir(void);
 void removeall(char*);
 
 static void
-rmtemp(void)
+cleanup(void)
 {
+	close(cout);
 	removeall(tmpdir);
 }
 
@@ -547,7 +548,7 @@ hostlinksetup(void)
 	// create temporary directory and arrange cleanup
 	if(tmpdir == nil) {
 		tmpdir = mktempdir();
-		atexit(rmtemp);
+		atexit(cleanup);
 	}
 
 	// change our output to temporary object file