[libgomp, testsuite, nvptx] Add libgomp.c/declare-variant-3-sm*.c

[libgomp, testsuite, nvptx] Add libgomp.c/declare-variant-3-sm*.c

Add openmp test-cases that test the omp declare variant construct:
...
  #pragma omp declare variant (f30) match (device={isa("sm_30")})
...
using the available nvptx isas.

Only the one for sm_30 is a dg-do run test-case, the other ones are dg-do
link.

Tested on x86_64 with nvptx accelerator.

libgomp/ChangeLog:

2022-02-24  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.c/declare-variant-3-sm30.c: New test.
	* testsuite/libgomp.c/declare-variant-3-sm35.c: New test.
	* testsuite/libgomp.c/declare-variant-3-sm53.c: New test.
	* testsuite/libgomp.c/declare-variant-3-sm70.c: New test.
	* testsuite/libgomp.c/declare-variant-3-sm75.c: New test.
	* testsuite/libgomp.c/declare-variant-3-sm80.c: New test.
	* testsuite/libgomp.c/declare-variant-3.h: New header file.