Add x86-64 implementation of cpu_set_tls() (like the kernel, we
just have to call do_arch_prctl() to set FS); this allows us to
enable NPTL.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>