linux-user/main.c: Remove redundant end_exclusive() in arm_kernel_cmpxchg64_helper()

start/end_exclusive() need be pairs, except the start_exclusive() in
stop_all_tasks() which is only used by force_sig(), which will be abort.
So at present, start_exclusive() in stop_all_task() need not be paired.

queue_signal() may call force_sig(), or return after kill pid (or queue
signal). If could return from queue_signal(), stop_all_task() would not
be called in time, the next end_exclusive() would be issue.

So in arm_kernel_cmpxchg64_helper() for ARM, need remove end_exclusive()
after queue_signal(). The related commit: "97cc7560 linux-user: Implement
new ARM 64 bit cmpxchg kernel helper".

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>