Detect all MIPS store instructions in cpu_signal_handler for all available
MIPS versions, and set is_write if encountering such store instructions.

This fixed the error while dealing with self-modified code for MIPS.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Kele Huang <kele.hwang@gmail.com>
Signed-off-by: Xu Zou <iwatchnima@gmail.com>
Message-Id: <20201002081420.10814-1-kele.hwang@gmail.com>
[rth: Use uintptr_t for pc to fix n32 build error.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>