bsd-user: assume pthreads and support of __thread

All compilers for some time have supported this. Follow linux-user and
eliminate the #define THREAD and unconditionally insert __thread where
needed. Please insert: "(see 24cb36a6: "configure: Make NPTL
non-optional")"

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>