添加rtthread相关代码

riscv/startup/start.S

@@ -0,0 +1,43 @@
+
+    .section .init
+    .globl _start
+    .type _start,@function
+
+.extern main
+# .extern trap_handler
+.extern trap_entry
+.extern entry
+_start:
+
+    la sp, _sp
+
+    /* Load data section */
+    la a0, _data_lma
+    la a1, _data
+    la a2, _edata
+    bgeu a1, a2, 2f
+1:
+    lw t0, (a0)
+    sw t0, (a1)
+    addi a0, a0, 4
+    addi a1, a1, 4
+    bltu a1, a2, 1b
+2:
+
+    /* Clear bss section */
+    la a0, __bss_start
+    la a1, _end
+    bgeu a0, a1, 2f
+1:
+    sw zero, (a0)
+    addi a0, a0, 4
+    bltu a0, a1, 1b
+2:
+    la a0, trap_entry
+    csrw mtvec, a0
+
+    /* Call global constructors */
+    # la a0, __libc_fini_array
+    # call __libc_init_array
+    # j main
+    j entry