The only "tls" support we have that's currently working is emulated tls, as exemplified by both clang and the gcc 4.9 port.
See /usr/src/lib/libcompiler_rt/emutls.c for how it works. It's not incredibly fast, but it does the job... Fortunately, the gcc and clang people managed to make some kind of standard, so both emutls flavors are actually compatible.
