[libgomp, testsuite, nvptx] Limit recursion in declare_target-{1,2}.f90

[libgomp, testsuite, nvptx] Limit recursion in declare_target-{1,2}.f90

When running testcases libgomp.fortran/examples-4/declare_target-{1,2}.f90 on
an RTX A2000 (sm_86) with driver 510.60.02 and with GOMP_NVPTX_JIT=-O0 I run
into:
...
FAIL: libgomp.fortran/examples-4/declare_target-1.f90 -O0 \
  -DGOMP_NVPTX_JIT=-O0 execution test
FAIL: libgomp.fortran/examples-4/declare_target-2.f90 -O0 \
  -DGOMP_NVPTX_JIT=-O0 execution test
...

Fix this by further limiting recursion depth in the test-cases for nvptx.

Furthermore, make the recursion depth limiting nvptx-specific.

Tested on x86_64 with nvptx accelerator.

libgomp/ChangeLog:

2022-04-01  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.fortran/examples-4/declare_target-1.f90: Define
	and use REC_DEPTH.
	* testsuite/libgomp.fortran/examples-4/declare_target-2.f90: Same.