Instead of relying on a build when TinyGo is being built, do it like all
other libraries when it is needed.
This brings a few benefits:
* No more running `make wasi-libc` on the command line as a special
case for WebAssembly. The generic `git submodule update --init` step
is now enough for wasi-libc.
* It becomes much easier to customize the build per system. For
example: include/exclude malloc as needed, disable/enable bulk
memory operations per target, etc.
Ayke van Laethem
632357ffb9