* introduce wasm32-freestanding-musl .h files to fix
conflicts with stddef.h and errno.h
* fix an issue with zig build system regarding installation of
webassembly libraries
* add implementations to zig's libc:
- strcmp
- strncmp
- strerror
- strlen
See #514
Unlike the other glibc source code checked into the repo, `csu/init.c`
did not have a license clause that allowed linking without restrictions.
`_IO_stdin_used` is the only symbol in the file and appears to be a 20
year old compatibility shim for the glibc 2.0 ABI. Obsolete in 2.1.