[add] laos using xv6-riscv thought

laos/src/kernel/entry.S

@@ -0,0 +1,20 @@
+#include "asm.h"
+
+.section .entry
+.global _entry
+_entry:
+        # set up a stack for C.
+        # stack0 is declared in start.c,
+        # with a 4096-byte stack per CPU.
+        # sp = stack0 + (hartid * 4096)
+        la.global $sp, stack0
+        lu12i.w $a0, (1024*4 >> 12) & 0xfffff
+        addi.w $a0, $a0, 1024*4 & 0xfff
+        csrrd $a1, CPUID
+        addi.w $a1, $a1, 1
+        mul.w $a0, $a0, $a1
+        add.w $sp, $sp, $a0
+        # jump to start() in start.c
+        bl start
+spin:
+        b spin