[nvptx, testsuite] Remove xpassing xfails for ptxas crashes

[nvptx, testsuite] Remove xpassing xfails for ptxas crashes

There's a couple of nvptx xfails that are related to ptxas crashes,
which are xpassing for me (with ptxas from cuda 9.1).

I've tested these on a range of cudas, from 5.0 to 11.1 and did not
manage to reproduce the fails.

Remove the xfails.  Tested on nvptx.

gcc/testsuite/ChangeLog:

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

	* gcc.c-torture/compile/pr42717.c: Remove nvptx xfail.
	* gcc.c-torture/compile/pr61684.c: Same.
	* gcc.c-torture/execute/pr20601-1.c: Same.
	* gcc.c-torture/execute/pr52129.c: Same.
	* gcc.c-torture/execute/pr59221.c: Same.
	* gcc.dg/pr68671.c: Same.