Develop a fully multithreaded C runtime for pthreads.

Several functions in the C runtime are currently effectively singlethreaded, because they carry global process specific state in JS code that can't be thread local. Therefore calls to these runtime functions from pthreads are proxied to the main thread. Migrating to use more musl can avoid this issue. Also, investigate creating a fully asm.js -based virtual filesystem to be used when targeting pthreads, so that C file I/O can be naturally multithreaded as well.