Building without threads support

[ncruces]

wasi-libc by default builds without thread support, and threads are a separate target wasm32-wasi-threads.
wasix-libc, apparently, always builds with thread support.

I had to patch big chunks of the Makefile (and some unrelated code) to build a sysroot with THREAD_MODEL='single' and produce a binary that didn't include WASM threads instructions (0xfe prefix).

Obviously, the code I'm trying to compile does not use the Pthreads API at all.

Is it a goal to support THREAD_MODEL='single'?