cmd/link: pass -Wl,-z,nodelete when creating an ELF shared library

Go shared libraries do not support dlclose, and there is no likelihood
that they will suppose dlclose in the future.  Set the DF_1_NODELETE
flag to tell the dynamic linker to not attempt to remove them from
memory.  This makes the shared library act as though every call to
dlopen passed the RTLD_NODELETE flag.

Fixes #12582.
Update #11100.
Update #12873.

Change-Id: Id4b6e90a1b54e2e6fc8355b5fb22c5978fc762b4
Reviewed-on: https://go-review.googlesource.com/15605Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

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

@@ -982,11 +982,14 @@ func hostlink() {
 		if HEADTYPE == obj.Hdarwin {
 			argv = append(argv, "-dynamiclib")
 		} else {
+			// ELF.
 			argv = append(argv, "-Wl,-Bsymbolic")
 			if UseRelro() {
 				argv = append(argv, "-Wl,-z,relro")
 			}
-			argv = append(argv, "-shared")
+			// Pass -z nodelete to mark the shared library as
+			// non-closeable: a dlclose will do nothing.
+			argv = append(argv, "-shared", "-Wl,-z,nodelete")
 		}
 	case BuildmodeShared:
 		// TODO(mwhudson): unless you do this, dynamic relocations fill