Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP'...

Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP' __builtin_is_initial_device: Fix effective-target keyword in 'libgomp.oacc-fortran/acc_on_device-2.f90'

The test case 'libgomp.oacc-fortran/acc_on_device-2.f90' added in
commit 3269a722
"Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP' __builtin_is_initial_device"
had a mismatch between dump file production and its scanning; the former needs
to use 'offload_target_nvptx' (like 'offload_target_amdgcn'), not
'offload_device_nvptx'.

	PR testsuite/82250
	libgomp/
	* testsuite/libgomp.oacc-fortran/acc_on_device-2.f90: Fix
	effective-target keyword.

libgomp/testsuite/libgomp.oacc-fortran/acc_on_device-2.f90

@@ -3,7 +3,7 @@
 ! Check whether 'acc_on_device()' is properly compile-time optimized. */
 
 ! { dg-additional-options "-fdump-tree-gimple -fdump-tree-optimized" }
-! { dg-additional-options -foffload-options=-fdump-tree-optimized { target { offload_device_nvptx || offload_target_amdgcn } } }
+! { dg-additional-options -foffload-options=-fdump-tree-optimized { target { offload_target_nvptx || offload_target_amdgcn } } }
 
 ! { dg-final { scan-tree-dump-times "acc_on_device" 1 "gimple" } }