Adding support for threadlocal variables for Wasm

[Aandreba]

Reasons

With the increased usage of Workers to add multi-threading capabilities to WebAssembly programs, I find myself missing thread local variables on zig Wasm.
Since Wasm is always assumed to be single-threaded, threadlocal variables are always treated as container-level variables.

Possible solutions

• Allow manually disabling the single_threaded build flag on Wasm targets (perhaps require shared_memory to be enabled).
• Add some mechanism to create thread local variables, even in single_threaded mode.

[andrewrk]

check out these latest developments:

• WASI: Implement experimental threading support #16207
• wasm-linker: finish shared-memory & TLS implementation #16439

I think there is nothing else to do here. Thread-local variables should work as you expect them to. If that is not the case, please provide some example code I'll re-open the issue.

[Aandreba]

Awesome!