target/mips: Move cpu_signal_handler definition around

We have 2 blocks guarded with #ifdef for sysemu, which
are simply separated by the cpu_signal_handler definition.

To simplify the following commits which involve various
changes in internal.h, first join the sysemu-guarded blocks.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-16-f4bug@amsat.org>