添加rtthread相关代码

riscv/startup/rtthread_irq.c

@@ -0,0 +1,15 @@
+#include "stddef.h"
+#include "head.h"
+#include "rtthread.h"
+
+void handle_trap(size_t mcause, size_t mepc, size_t sp) {
+  my_printf("interupt mcause=%08x, mepc=%08x, sp=%08x\n", mcause, mepc, sp);
+}
+
+void rt_hw_board_init(void) {
+
+}
+
+// rt_device_t rt_device_find(const char* name) {
+//   return 0;
+// }