target-moxie: set do_interrupt to a target-specific helper function

The value of "do_interrupt" member of CPUClass shoule be set to a
target-specific function, or it will lead to a segfault like below:

$ moxie-softmmu/qemu-system-moxie -M moxiesim
Segmentation fault

Cc: Anthony Green <green@moxielogic.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Dunrong Huang <huangdr@cloud-times.com>
Signed-of-by: Anthony Green <green@moxielogic.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>