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std: Implement TLS for wasm32-unknown-unknown
This adds an implementation of thread local storage for the `wasm32-unknown-unknown` target when the `atomics` feature is implemented. This, however, comes with a notable caveat of that it requires a new feature of the standard library, `wasm-bindgen-threads`, to be enabled. Thread local storage for wasm (when `atomics` are enabled and there's actually more than one thread) is powered by the assumption that an external entity can fill in some information for us. It's not currently clear who will fill in this information nor whose responsibility it should be long-term. In the meantime there's a strategy being gamed out in the `wasm-bindgen` project specifically, and the hope is that we can continue to test and iterate on the standard library without committing to a particular strategy yet. As to the details of `wasm-bindgen`'s strategy, LLVM doesn't currently have the ability to emit custom `global` values (thread locals in a `WebAssembly.Module`) so we leverage the `wasm-bindgen` CLI tool to do it for us. To that end we have a few intrinsics, assuming two global values: * `__wbindgen_current_id` - gets the current thread id as a 32-bit integer. It's `wasm-bindgen`'s responsibility to initialize this per-thread and then inform libstd of the id. Currently `wasm-bindgen` performs this initialization as part of the `start` function. * `__wbindgen_tcb_{get,set}` - in addition to a thread id it's assumed that there's a global available for simply storing a pointer's worth of information (a thread control block, which currently only contains thread local storage). This would ideally be a native `global` injected by LLVM, but we don't have a great way to support that right now. To reiterate, this is all intended to be unstable and purely intended for testing out Rust on the web with threads. The story is very likely to change in the future and we want to make sure that we're able to do that!
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