target/mips: Advance pc after semihosting exception

Delay generating the exception until after we know the
insn length, and record that length in env->error_code.

Fixes: 8ec7e3c5 ("target/mips: Use an exception for semihosting")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1126


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