target/mips: Use an exception for semihosting

Within do_interrupt, we hold the iothread lock, which
is required for Chardev access for the console, and for
the round trip for use_gdb_syscalls().

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