musl now (as of Sep 2014-ish) implements C11 threads!
According to this recent musl mailing list post and other conversation on that list in Jul-Sep 2014, it appears that the musl library now (as of 2014-09-07) implements the C11 threads interface, though the comparison chart; does not as of this writing reflect this.
Since this question is re-opened, I moved this content to a proper answer.