[libgomp, testsuite, nvptx] Fix pr96390.c without CUDA

When running the libgomp testsuite on x86_64 with nvptx accelerator, we run into:
...
XPASS: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
FAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c execution test
...

The problem is that we're expecting the following ptxas error:
...
XFAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
Excess errors:
ptxas /tmp/ccZYDw8N.o, line 90; error   : Call to 'baz' requires call prototype
ptxas /tmp/ccZYDw8N.o, line 90; error   : Unknown symbol 'baz'
...

But it's not triggered because ptxas is not in the path, so nvptx-none-as
defaults to --no-verify.

So instead, we run into the same error at execution time.

Fix this by forcing verification using:
...
/* { dg-additional-options "-foffload=-Wa,--verify" \
     { target offload_target_nvptx } } */
...
such that we run into the xfail in this way instead:
...
XFAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
Excess errors:
nvptx-as: error trying to exec 'ptxas': execvp: No such file or directory
nvptx-as: ptxas returned 255 exit status
...

Tested on x86_64-linux with nvptx accelerator.

libgomp/ChangeLog:

2022-02-21  Tom de Vries  <[email protected]>

	PR testsuite/104146
	* testsuite/libgomp.c++/pr96390.C: Add additional-option
	-foffload=-Wa,--verify for nvptx.
	* testsuite/libgomp.c-c++-common/pr96390.c: Same.

Author: vries

libgomp/testsuite/libgomp.c++/pr96390.C

@@ -1,4 +1,5 @@
 /* { dg-additional-options "-O0 -fdump-tree-omplower" } */
+/* { dg-additional-options "-foffload=-Wa,--verify" { target offload_target_nvptx } } */
 /* { dg-xfail-if "PR 97106/PR 97102 - .alias not (yet) supported for nvptx" { offload_target_nvptx } } */
 
 #include <cstdlib>

libgomp/testsuite/libgomp.c-c++-common/pr96390.c

@@ -1,4 +1,5 @@
 /* { dg-additional-options "-O0 -fdump-tree-omplower" } */
+/* { dg-additional-options "-foffload=-Wa,--verify" { target offload_target_nvptx } } */
 /* { dg-require-alias "" } */
 /* { dg-xfail-if "PR 97102/PR 97106 - .alias not (yet) supported for nvptx" { offload_target_nvptx } } */