[nvptx, testsuite] Remove dg-skip-if for ptxas crash fixed in cuda 8.0

[nvptx, testsuite] Remove dg-skip-if for ptxas crash fixed in cuda 8.0

The test-case gcc.c-torture/execute/pr68185.c has a dg-skip-if for nvptx due
to "ptxas crashes or executes incorrectly".

I've managed to reproduce the problem using cuda 7.5:
...
FAIL: gcc.c-torture/execute/pr68185.c   -O0  (test for excess errors)
Excess errors:
nvptx-as: ptxas terminated with signal 11 [Segmentation fault], core dumped
...
but it's fixed starting cuda 8.0.

Remove the dg-skip-if.  Tested on nvptx.

gcc/testsuite/ChangeLog:

2020-10-15  Tom de Vries  <tdevries@suse.de>

	* gcc.c-torture/execute/pr68185.c: Remove dg-skip-if for nvptx.