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.