[libgomp/testsuite] Fix libgomp.fortran/examples-4/declare_target-{1,2}.f90

[libgomp/testsuite] Fix libgomp.fortran/examples-4/declare_target-{1,2}.f90

The test-cases libgomp.fortran/examples-4/declare_target-{1,2}.f90 mean to
set an nvptx-specific limit using offload_target_nvptx, but also change
behaviour for amd.

That is, there is now a difference in behaviour between:
- a compiler configured for GCN offloading, and
- a compiler configured for both GCN and nvptx offloading.

Fix this by using instead on_device_arch_nvptx.

Tested on x86_64 with nvptx accelerator.

libgomp/ChangeLog:

2022-04-04  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.fortran/examples-4/declare_target-1.f90: Use
	on_device_arch_nvptx instead of offload_target_nvptx.
	* testsuite/libgomp.fortran/examples-4/declare_target-2.f90: Same.