使得DragonOS kernel 能为riscv64编译通过(尚未能启动) (#457)

* 使得DragonOS kernel 能为riscv64编译通过(尚未能启动)

* 修正了系统调用号声明不正确的问题，同时添加了编译配置文档

kernel/src/arch/riscv64/mod.rs

@@ -0,0 +1,21 @@
+pub mod asm;
+pub mod cpu;
+pub mod interrupt;
+pub mod ipc;
+mod kvm;
+pub mod mm;
+pub mod msi;
+pub mod pci;
+pub mod pio;
+pub mod process;
+pub mod rand;
+pub mod sched;
+pub mod syscall;
+pub mod time;
+
+pub use self::interrupt::RiscV64InterruptArch as CurrentIrqArch;
+pub use self::kvm::RiscV64KVMArch as KVMArch;
+pub use self::mm::RiscV64MMArch as MMArch;
+pub use self::pci::RiscV64PciArch as PciArch;
+pub use self::pio::RiscV64PortIOArch as CurrentPortIOArch;
+pub use self::time::RiscV64TimeArch as CurrentTimeArch;