tcg/mips: Make direct jump patching thread-safe

Ensure direct jump patching in MIPS is atomic by using
atomic_read()/atomic_set() for code patching.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Message-Id: <1461341333-19646-11-git-send-email-sergey.fedorov@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
[rth: Merged the deposit32 followup.]
[rth: Merged the following followup.]
Message-Id: <1462210518-26522-1-git-send-email-sergey.fedorov@linaro.org>