This symbol isn't actually used anywhere, so it's fine to be renamed.

Once we are at MSRV 1.37, we can change the code to use an
[unnamed `const`](https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#using-unnamed-const-items-for-macros),
for example:
```rust
const _: () = {
    // We use Rust ABI to be safe against potential panics in the passed function.
    #[no_mangle]
    unsafe fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
```