Fix calling prototyped functions via function pointers

Calling a prototyped function via a function pointer with the right
prototype doesn't work correctly, if the called function requires
argument coercion...  Like, e.g., with:

  float mult (float f1, float f2) { return f1 * f2; }

  (gdb) p mult (2, 3.5)
  $1 = 7
  (gdb) p ((float (*) (float, float)) mult) (2, 3.5)
  $2 = 0

both calls should have returned the same, of course.  The problem is
that GDB misses marking the type of the function pointer target as
prototyped...

Without the fix, the new test fails like this:

 (gdb) p ((int (*) (float, float)) t_float_values2)(3.14159,float_val2)
 $30 = 0
 (gdb) FAIL: gdb.base/callfuncs.exp: p ((int (*) (float, float)) t_float_values2)(3.14159,float_val2)

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* gdbtypes.c (lookup_function_type_with_arguments): Mark function
	types with more than one parameter as prototyped.

gdb/testsuite/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* gdb.base/callfuncs.exp (do_function_calls): New parameter
	"prototypes".  Test calling float functions via prototyped and
	unprototyped function pointers.
	(perform_all_tests): New parameter "prototypes".  Pass it down.
	(top level): Pass down "prototypes" parameter to
	perform_all_tests.

gdb/gdbtypes.c

@@ -547,6 +547,8 @@ lookup_function_type_with_arguments (struct type *type,
 	  gdb_assert (nparams == 0);
 	  TYPE_PROTOTYPED (fn) = 1;
 	}
+      else
+	TYPE_PROTOTYPED (fn) = 1;
     }
 
   TYPE_NFIELDS (fn) = nparams;