Also remove all the wasi-libc files we used to ship, but never compile.
The latest wasi-libc HEAD has an extra commit (a6f871343313220b76009827ed0153586361c0d5), which makes preopen initialization lazy.
Unfortunately, that breaks quite a lot of things on our end. Applications now need to explicitly call __wasilibc_populate_preopens() everywhere when the libc is linked. That can wait after 0.11.
* Update wasi-libc to a00bf321eeeca836ee2a0d2d25aeb8524107b8cc
It includes a port of emscripten's allocator that performs
performs much better than the old one.
Most importantly, it includes the prerequisites to later add
support for POSIX threads.
