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

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 3269a722b7a03613e9c4e2862bc5088c4a17cc11
"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.