Add support for "fsm_export" synthesis attributes to fsm_export pass

This allows to specify the file name for exported files directly in the HDL
source via the fsm_export=... attribute on the FSM state register.

Verilog example:
    (* fsm_export="my_fsm.kiss2" *)
    reg [3:0] state;

The fsm_export pass now also accepts the option "-noauto". This causes only
FSMs with the fsm_export attribute to be exported, any other FSMs are ignored.