Prevent optimizing out of benchmark function call

Resolves: #15424

The compiler would optimize the benchmark function call out of the
loop and call it only once, resulting in blazingly fast times for some
benchmarks (notably atan, sin and cos).  Mark the inputs as volatile
so that the code is forced to read again from the input for each
iteration.

scripts/bench.pl

@@ -71,7 +71,7 @@ else {
       $decl = "$decl,";
     }
 
-    $arg_struct = "$arg_struct $arg arg$num;";
+    $arg_struct = "$arg_struct volatile $arg arg$num;";
     $bench_func = "$bench_func variants[v].in[i].arg$num";
     $decl = "$decl $arg";
     $num = $num + 1;