Remove unused file.
Let libc.symbols use same symbol format as libcxx.

This is just the output of 'llvm-nm -extern-only -defined-only'.
Add musl wchar function implementations.
Add musl multibyte function implementations.
Move wchar,mb stuff to libcextra.
Bump version.
Add a readme giving the date / revision of these sources.
Clean up libc / libcxx build infrastructure.

This creates a couple of additional methods to reduce copy/paste
and make things a bit easier to follow.
Merge pull request #1063 from waywardmonkeys/musl-libc

Add wchar and multibyte libc functions