Unfortunately some duplicate files must remain in lib/libc/wasi/libc-top-half
because they include internal headers *in the same directory* which have edits
relative to upstream musl. Because C is an amazing language, there is no way to
make it so that e.g. upstream musl's src/stdio/fputc.c includes wasi-libc's
src/stdio/putc.h instead of the upstream putc.h. The preprocessor always
searches the current directory first for quote includes.
Anyway, this still takes us from 2.9M to 1.4M for the combination of
lib/libc/wasi and lib/libc/include/wasm-wasi-musl, so I still call it a win.
* 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.
Rename include dir to match the convention:
from `wasm32-wasi` to `wasm-wasi-musl`
Add building stubs which will be used to build and cache WASI
libc sysroot.
