PRU-GCC bugfix and initial version of a simulator

- Fix for a critical pru-gcc bug in signed comparison code generation (issue #1 in github).
- Add LOOP/ILOOP instructions to GAS.
- Initial version of PRU ISA simulator. There is no I/O support, so it will be useful mainly for verifying and testing pru-gcc.

Signed-off-by: Dimitar Dimitrov <dinuxbg@gmail.com>