cmd/link: increase ELFRESERVE to a full page

Etcd and kubernetes have hit this.
See  https://bugzilla.redhat.com/show_bug.cgi?id=1248071

Change-Id: I6231013efa0a19ee74f7ebacd1024adb368af83a
Reviewed-on: https://go-review.googlesource.com/12951Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/elf.go

@@ -713,7 +713,7 @@ var Elfstrdat []byte
  * On FreeBSD, cannot be larger than a page.
  */
 const (
-	ELFRESERVE = 3072
+	ELFRESERVE = 4096
 )
 
 /*