Use cpu_m68k_get_ccr()/cpu_m68k_set_ccr() to setup and restore correctly
the value of SR in the context structure. Fix target_rt_setup_ucontext().

Fixes: 3219de45 ("linux-user: correctly manage SR in ucontext")
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-Id: <20180104012913.30763-4-laurent@vivier.eu>