risc-v的寻址模式

1. Immediate addressing, where the operand is a constant within the
instruction itself.
2. Register addressing, where the operand is a register.
3. Base or displacement addressing, where the operand is at the
memory location whose address is the sum of a register and a
constant in the instruction.
4. PC-relative addressing, where the branch address is the sum of the
PC and a constant in the instruction.