Implement WebAssembly thread-local support for WASIX

[john-sharratt]

Goal

Currently wasix uses syscalls for getting and setting local thread variables which adds quite some overhead - instead the TLS area should be used for this thus avoiding this overhead and removing some redundant syscalls.

Impacted syscalls

The syscalls impacted are as follows:

        "thread_local_create" => Function::new_typed_with_env(&mut store, env, thread_local_create::<Memory32>),
        "thread_local_destroy" => Function::new_typed_with_env(&mut store, env, thread_local_destroy),
        "thread_local_set" => Function::new_typed_with_env(&mut store, env, thread_local_set),
        "thread_local_get" => Function::new_typed_with_env(&mut store, env, thread_local_get::<Memory32>),

Rust Standard Library

The rust standard library must be modified in order to use TLS instead of the wasix syscalls - the locations of the thread local implementation is as follows:

https://github.com/wasmerio/rust/blob/wasix/library/std/src/thread/local.rs
https://github.com/wasmerio/rust/blob/wasix/library/std/src/sys/wasix/atomics/thread_local_key.rs

Avoiding memory leaks

Particular attention must be made that when threads exit they do not cause memory leaks, at present this is done by creating a callback that the runtime executes after the thread exits _thread_local_destroy. This method will free any memory used by the local thread variables including invoking any destructors (for unsafe allocations). One could use the pthreads libc implementation to achieve the same thing rather than having this as a runtime responsibility.

wasix-libc

Threading is currently implemented using pthreads in the libc fork here:
https://github.com/wasmerio/wasix-libc

One approach to solving this problem would be to implement rust local thread storage entirely using the pthreads implementation however this is not the only way approach and a fully rust native solution is also possible.

Cargo build

WASIX applications can be compiled using this helpful toolchain fork:
https://github.com/wasmerio/cargo-wasix

Testing

There is a fully working http server that uses local thread storage extensively and thus can be used for functional and non-functional tests - it is located here:
https://github.com/john-sharratt/static-web-server

[theduke]

After this is implemented in the WASIX toolchain, support for the custom TLS related syscalls should be removed from the wasix crate.

[ptitSeb]

TODO: remove the old syscalls

[ptitSeb]

Tentative v4.0

[theduke]

Leftover wasix exports were removed in #3906 .