#487 Add thumbv4t-none-eabi to the list of targets that don't support…

[wtetzner]

Add thumbv4t-none-eabi to the list of targets that don't support atomics.

With this change, it builds fine for thumbv4t-none-eabi.

Without this change, I get a build failure for thumbv4t-none-eabi:

    Blocking waiting for file lock on package cache
    Updating crates.io index
    Blocking waiting for file lock on package cache
   Compiling log v0.4.14
   Compiling cfg-if v1.0.0
error[E0599]: no method named `compare_exchange` found for struct `AtomicUsize` in the current scope
    --> /Users/walter/.cargo/registry/src/github.com-1ecc6299db9ec823/log-0.4.14/src/lib.rs:1347:33
     |
1347 |     let old_state = match STATE.compare_exchange(
     |                                 ^^^^^^^^^^^^^^^^ method not found in `AtomicUsize`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `log` due to previous error

[KodrAus]

Thanks @wtetzner!