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

Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP' __builtin_is_initial_device: Revert 'gimple_fold_builtin_acc_on_device' change

The motivation of the 'gimple_fold_builtin_acc_on_device' change in
commit 3269a722
"Fortran: Use OpenACC's acc_on_device builtin, fix OpenMP' __builtin_is_initial_device"
is unclear, and it unnecessarily diverges GCC's (default)
'--disable-offload-targets' vs. '--enable-offload-targets=[...]'
configurations.

	PR testsuite/82250
	gcc/
	* gimple-fold.cc (gimple_fold_builtin_acc_on_device): Revert last
	change.
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/routine-nohost-1.c: Revert
	last change.