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: Harmonize 'libgomp.oacc-fortran/acc_on_device-1-*'

The test case 'libgomp.oacc-fortran/acc_on_device-1-1.f90' added in
commit 3269a722b7a03613e9c4e2862bc5088c4a17cc11
"Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP' __builtin_is_initial_device"
was missing '-fno-builtin-acc_on_device', and all
'libgomp.oacc-fortran/acc_on_device-1-*' need comments, why that option is
specified.

	PR testsuite/82250
	libgomp/
	* testsuite/libgomp.oacc-fortran/acc_on_device-1-1.f90: Add
	'-fno-builtin-acc_on_device'.
	* testsuite/libgomp.oacc-fortran/acc_on_device-1-2.f: Comment.
	* testsuite/libgomp.oacc-fortran/acc_on_device-1-3.f: Comment.