riscv: small refactoring for add_memory_int32/64

Currently, add_memory_int32/64 for riscv can only add a sign-extended 12-bit immediate to memory since they call addi/addiw assembler direcly.

This constraint could be relaxed when the given memory address is in the expected form: base register plus a sign-extended 12-bit offset. In this case, we can emit code for load + add/sub + store sequence adding arbitrary immediate to memory with no more than two scratch registers (t0 and t1) available.

We could also refactor these two functions into four seperate functions: increment, incrementw, decrement and decrementw, so that it will be more clear in code logic at the call sites.