openmp: Fix libgomp.c++ testsuite errors for non-offload configs

Some testcases for libgomp.c++ only works for non-shared address space offloading,
because it exercises the zero-length array section behavior for offloaded
address space, testing for NULL/non-NULL cases.

libgomp/ChangeLog:

	* testsuite/libgomp.c++/target-lambda-1.C: Only run under
	"target offload_device_nonshared_as"
	* testsuite/libgomp.c++/target-this-3.C: Likewise.
	* testsuite/libgomp.c++/target-this-4.C: Likewise.

libgomp/testsuite/libgomp.c++/target-lambda-1.C

@@ -1,3 +1,5 @@
+// { dg-do run { target offload_device_nonshared_as } }
+
 #include <cstdlib>
 #include <cstring>
 

libgomp/testsuite/libgomp.c++/target-this-3.C

@@ -1,3 +1,5 @@
+// { dg-do run { target offload_device_nonshared_as } }
+
 #include <stdio.h>
 #include <string.h>
 extern "C" void abort ();

libgomp/testsuite/libgomp.c++/target-this-4.C

@@ -1,6 +1,7 @@
-
 // We use 'auto' without a function return type, so specify dialect here
 // { dg-additional-options "-std=c++14" }
+// { dg-do run { target offload_device_nonshared_as } }
+
 #include <cstdlib>
 #include <cstring>