Unlike other architectures, on x86 (but not x86-64), CLONE_SETTLS
takes a pointer to a struct user_desc instead of a pointer to the
TLS itself. Rather than have to deal with this here, let's just use
the old __set_tls mechanism we used to use (and still use for the
main thread on all architectures, so it's not going away any time
soon).
Bug: 11826724
Change-Id: I02a27939a73ae6cea1134a3f4c1dd7eafea479da
80906141f7