riscv: Fix broken MacroAssembler::debug64

MacroAssembler::stop() and StubGenerator::generate_verify_oop() would first push all regs (calling MacroAssembler::pusha()) onto the stack and then call MacroAssembler::debug64 to print the pushed regs. But MacroAssembler::pusha() won't push x0~x4 so the result of MacroAssembler::debug64() is broken.