RISC-V: Use macro-assembler functions as appropriate

Use of assember functions on riscv can be improved. For example, we use 'fence(0xf, 0xf)' in function TemplateInterpreterGenerator::generate_safept_entry_for to emit a full memory fence instruction. Here we should use 'membar(MacroAssembler::AnyAny)' instead, which would make the code more readable. We should also replace the calls to 'li' with 'mv' so that it will be more consistent when moving integer constant.