execution bugfixes

[fmoletta]

Fix the following bugs:

• JALR: incorrect PC update (+offset should override usual +4)
• Store/Load: used register address instead of value at register as base
• JALR/JAL: if dst is zero, do not write next pc into it
• MultipleOpcodes: Use i32 for immediate
• Arithmetic operations with immediate: implement missing operations
• SP: set starting sp value according to current size constraints