It turns out we were already allocating static address space for TLS
data along with the non-TLS static data, but this space was going
With this change, we include the TLS segment in __wasm_init_memory
(which does the work of loading the passive segments into memory when a
module is first loaded). We also set the __tls_base global to point
to the start of this segment.
This means that the runtime can use this static copy of the TLS data for
the first/primary thread if it chooses, rather than doing a runtime
allocation prior to calling __wasm_init_tls.
Practically speaking, this will allow emscripten to avoid dynamic
allocation of TLS region on the main thread.
Part of work towards https://github.com/emscripten-core/emscripten/issues/16948