[libgomp] In OpenACC testing, by default only build for the offload target that we're actually going to test

... to avoid compilation overhead, and to keep simple '-foffload=[...]'
handling in test cases.

	libgomp/
	* testsuite/libgomp.oacc-c++/c++.exp: Specify
	"-foffload=$offload_target".
	* testsuite/libgomp.oacc-c/c.exp: Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.
	* testsuite/lib/libgomp.exp
	(check_effective_target_openacc_nvidia_accel_configured): Remove,
	as (conceptually) merged into
	check_effective_target_openacc_nvidia_accel_selected.  Adjust all
	users.

From-SVN: r269109