[lld][WebAssembly] Add symbols marking start/end of stack region

Currently emscripten is make assumptions about that memory layout,
assuming the stack is between `__data_end` and `__heap_base`:

https://github.com/emscripten-core/emscripten/blob/af961ad5c4c278ec510f0b7f7d522a95ee5a90f8/system/lib/compiler-rt/stack_limits.S#L42-L61

With this change we can be more precise:

https://github.com/emscripten-core/emscripten/pull/18057

Differential Revision: https://reviews.llvm.org/D135910