Does any C library implement C11 threads for GNU/Linux?
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.
Finally GNU LIBC 2.28 supports C11 ISO Threads.
I assume Archlinux, Gentoo and Suse Tumbleweed will support it quickly. Later in this year Fedora and Ubuntu and the others in 2019 or later.
https://github.com/jtsiomb/c11threads is a "Trivial C11 threads.h implementation over POSIX threads." contained in a single header.