[nvptx, testsuite] Remove dg-skip-if for ptxas oom

[nvptx, testsuite] Remove dg-skip-if for ptxas oom

There's a dg-skip-if for nvptx that is related to ptxas oom.

If I remove it, the test-case passes for me (with ptxas from cuda 9.1).

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

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

gcc/testsuite/ChangeLog:

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

	* gcc.c-torture/compile/limits-externdecl.c: Remove dg-skip-if for
	nvptx.