target-microblaze: Break out trap_userspace()

target-microblaze: Break out trap_userspace()

Break out trap_userspace() to avoid open coding it everywhere.
For privileged insns, we now always stop translation of the
current insn for cores without exceptions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>