This set of commits implements initial pthreads library support for Emscripten code, and allows native code to utilize multithreading with the pthreads API. This backs on experimental research on the JavaScript SharedArrayBuffer and Atomics APIs from https://docs.google.com/document/d/1NDGA_gZJ7M7w1Bh8S0AoDyEqwDdRh4uSoTPSNn77PFk/edit?usp=sharing , and as such is preliminary and unsupported in othe