meta: Allow a clippy lint that appears in recent versions

We are now getting this warning:

    warning: the variable `j` is used as a loop counter
       --> libm/src/math/rem_pio2_large.rs:268:5
        |
    268 |     for i in 0..=m {
        |     ^^^^^^^^^^^^^^ help: consider using: `for (j, i) in ((jv as i32) - (jx as i32)..).zip((0..=m))`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_counter_loop
        = note: `#[warn(clippy::explicit_counter_loop)]` on by default

    warning: `libm` (lib) generated 1 warning

Where the suggestion is definitely not an improvement.

library/compiler-builtins/libm/src/math/rem_pio2_large.rs

@@ -1,4 +1,6 @@
 #![allow(unused_unsafe)]
+// FIXME(clippy): the suggested fix is bad but the code as-written could be better
+#![allow(clippy::explicit_counter_loop)]
 /* origin: FreeBSD /usr/src/lib/msun/src/k_rem_pio2.c */
 /*
  * ====================================================