wasi-libc: Deduplicate sources and headers with regards to upstream musl.

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.

Author: alexrp