[lld][WebAssembly] Allow use of statically allocated TLS region.

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
unused/ignored.

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.

Differential Revision: https://reviews.llvm.org/D126107