Add 'gcc.target/nvptx/alias-weak-1.c'

... testing for the GCC/nvptx "weak alias definitions not supported" error
diagnostic (limitation of PTX).

	gcc/testsuite/
	* gcc.target/nvptx/alias-weak-1.c: New.

gcc/testsuite/gcc.target/nvptx/alias-weak-1.c

+/* { dg-do compile } */
+/* { dg-add-options ptx_alias } */
+
+void __f ()
+{
+}
+
+void f () __attribute__ ((weak, alias ("__f")));
+/* { dg-error {weak alias definitions not supported} {} { target *-*-* } .-1 }
+   (limitation of PTX).  */