runtime: fix unsync_load on atomic types

[Darksonn]

The unsync_load methods currently create a mutable reference to the atomic to perform the read. However, this is not allowed as it will sometimes race with other atomic loads. This occasionally causes the stress1 test to fail with the following error:

error: Undefined Behavior: not granting access to tag <3613730> because that would remove [Unique for <3613708>] which is protected because it is an argument of call 1030169
   --> tokio/src/runtime/scheduler/multi_thread/queue.rs:386:28
    |
386 |             let src_tail = self.0.tail.load(Acquire);
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <3613730> because that would remove [Unique for <3613708>] which is protected because it is an argument of call 1030169
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <3613730> was created by a SharedReadWrite retag at offsets [0x20..0x24]
   --> tokio/src/runtime/scheduler/multi_thread/queue.rs:386:28
    |
386 |             let src_tail = self.0.tail.load(Acquire);
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^
help: <3613708> is this argument
   --> tokio/src/loom/std/atomic_u32.rs:26:10
    |
26  |         *(*self.inner.get()).get_mut()
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: BACKTRACE:
    = note: inside `runtime::scheduler::multi_thread::queue::Steal::<runtime::blocking::schedule::NoopSchedule>::steal_into2` at tokio/src/runtime/scheduler/multi_thread/queue.rs:386:28
note: inside `runtime::scheduler::multi_thread::queue::Steal::<runtime::blocking::schedule::NoopSchedule>::steal_into` at tokio/src/runtime/scheduler/multi_thread/queue.rs:348:21
   --> tokio/src/runtime/scheduler/multi_thread/queue.rs:348:21
    |
348 |         let mut n = self.steal_into2(dst, dst_tail);
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at tokio/src/runtime/tests/queue.rs:132:20
   --> tokio/src/runtime/tests/queue.rs:132:20
    |
132 |                 if steal.steal_into(&mut local, &mut metrics).is_some() {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace