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: qemu-project/qemu#1126


Signed-off-by: Richard Henderson <richard.henderson@linaro.org>