Add R1(return val) to reserved register set.

This makes the compiler not to optimize away R1 from return path.
Otherwise the compilation was failing in -O1 or greater.
Lets use // as comment in the assembly
Add isCall property to CALL
Remove delaySlot property from RET(TeeRISC doesnt have any delaySlot).
Added basic support for generating ELF file output.
Added ZERO, SP and FP registers to getRegisterNumbering()
Added ADD_IMM instruction

Fixed emitPrologue() and emitEpilogue() to use ADD_IMM instead of ADD
instruction when adding a register with immediate.