Close https://github.com/WebAssembly/wasi-libc/issues/520

Add FTS implementation derived from musl-fts with a few modifications.
The compiled fts.o is included in the libc.a archive, and the fts.h
header is installed in the sysroot (`include/fts.h`).

* fts/musl-fts: Add a copy of the musl-fts sources with modifications.
* fts/patches: A set of patches to apply to the musl-fts sources.
* Upstream pull request: https://github.com/void-linux/musl-fts/pull/14
* fts/update-musl-fts.sh: A script to update the musl-fts sources with
the patches applied.
* fts/config.h: A configuration header included by the musl-fts sources.
* test/smoke: Add a test suite for wasi-libc specific features that
libc-test does not cover.