Emscripten does have a stand alone linker, wasm-ld. This patch adds the
linker, adds detection for the linker, and removes the IsLinkerMixin for
emscripten. This is a little more correct, and makes the code a lot
cleaner and more robust.
Emscripten has pthread support (as well as C++ threads), but we don't
currently implement them. This fixes that by adding the necessary code.
The one thing I'm not sure about is setting the pool size. The docs
suggest that you really want to do this to ensure that your code works
correctly, but the number should really be configurable, not sure how to
set that.
Fixes#6684
Dylan Baker
Daniel Mensinger
Wolfgang Stöggl
Andrei Alexeyev