Build error on MIPS 32bit

[faulesocke]

error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
 --> [...]/async-std-1.4.0/src/task/task_id.rs:2:25
  |
2 | use std::sync::atomic::{AtomicU64, Ordering};
  |                         ^^^^^^^^^
  |                         |
  |                         no `AtomicU64` in `sync::atomic`
  |                         help: a similar name exists in the module: `AtomicU8

Unfortunately, according to the docs, 64bit atomic types are not available on 32bit mips architectures.

I've looked into the source location and two solutions to this come to my mind:

1. use AtomicU32. This might or might not be okay on 32bit architectures.
2. Rewrite this (for mips only) with blocking logic (i.e. keep the counter inside a mutex)

If someone tells me which one is the preferred solution, I can write a PR.

[yoshuawuyts]

Related: #286

[faulesocke]

Yep, this looks like it will solve my problem. Any estimations on when it might be ready?

[yoshuawuyts]

@faulesocke this will mostly depend on availability of our team; we'll be sitting down to plan next steps for async-std next week, I'll be sure to bring this up.

[faulesocke]

It looks like #286 was closed without being merged. Are there other plans to solve this?