- Add new instructions such as: clz, ctz, popcnt.

- See if it is worth exporting mul2, mulu2, div2, divu2. 

- Support of globals saved in fixed registers between TBs.

Ideas:

- Move the slow part of the qemu_ld/st ops after the end of the TB.

- Change exception syntax to get closer to QOP system (exception
  parameters given with a specific instruction).

- Add float and vector support.
