Adding processor specific extensions

Replaced the _andn_ instruction with two processor specific instructions _swi_ and _ext_.

_swi_ performs the software interrupt specified in the lower 8-bits. This allows portable interaction with an operating system or other environment and should help by providing hooks for debugging.

_ext_ is a normally undefined instruction with the purpose of providing a method of adding extensions to the instruction set. The decoding of the lower 11-bits is specific to the processor's implementation. An example extension might be to add additional two register opcodes in the lower 5 bits similarly to MIPS/16^3. However, the extension opcode should not be used by portable applications and a simple implementation may just ignore/halt the instruction.