openmp: Fix g++.dg/gomp/declare-target-3.C testcase when offloading is disabled

2020-12-18  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/testsuite/
	* g++.dg/gomp/declare-target-3.C: Only check .offload_var_table
	entries if offloading is enabled.