[nvptx, testsuite] Disable default xfails

[nvptx, testsuite] Disable default xfails

For nvptx, I run into a couple of XPASSes due to nvptx not being listed in:
...
  { xfail { ! { long list of targets } } }
...

Add nvptx to the list to get a PASS.  Tested on nvptx.

gcc/testsuite/ChangeLog:

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

	* gcc.dg/attr-alloc_size-11.c: Don't xfail for nvptx.
	* gcc.dg/tree-ssa/20040204-1.c: Same.