wasm: Finish TLS & shared-memory implementation #68
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Fixes memory_init by making timeout a i64 instead of i32. Also fixes tls_init by setting the __tls_base global instead of retrieving it and correctly writing the 0 byte as u8 rather than as u32.
When we have shared-memory, the memory will be initialized during module initialization. To ensure this is being done, we set the mentioned function as the entrypoint of the WebAssembly module. This function will be called upon thread creation too, to ensure the memory is correctly initialized. The execution of the first startup will then continue with `__start` (when available).
We were writing the values as unsigned integers, causing OOB memory reads. We now correctly write the constants as signed integers by first bitcasting them.
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This implements TLS relocations and fixes various issues in the shared-memory implementation. With these additions, we can now correctly link the threading example from the Zig main repo. The last remaining step for full static-linking support is garbage collection.