nvptx: Specify '-mno-alias' for 'gcc.dg/pr60797.c' [PR60797, PR104957]

2014 Subversion r209299 (Git commit 8330537b)
"Fix PR60797" added this test case, which we now amend so that it's able to
test its thing also in '--target=nvptx-none' configurations with symbol alias
support enabled (..., and test nvptx '-mno-alias').

	PR middle-end/60797
	PR target/104957
	gcc/testsuite/
	* gcc.dg/pr60797.c: For nvptx, specify '-mno-alias'.

gcc/testsuite/gcc.dg/pr60797.c

-/* { dg-do compile } */
-/* { dg-skip-if "" { alias } } */
+/* If there's support for symbol aliases, have to 'dg-skip-if' -- unless
+   there's a way to disable this support.
+   { dg-additional-options -mno-alias { target nvptx-*-* } }
+   { dg-skip-if "" { { ! nvptx-*-* } && alias } } */
 
 extern int foo __attribute__((alias("bar"))); /* { dg-error "supported" } */
 int main()