Switch ok with debug mode apps, implement sys_exit correctly later.

os/src/task/switch.S

@@ -0,0 +1,34 @@
+.altmacro
+.macro SAVE_SN n
+    sd s\n, (\n+1)*8(sp)
+.endm
+.macro LOAD_SN n
+    ld s\n, (\n+1)*8(sp)
+.endm
+    .section .text
+    .globl __switch
+__switch:
+    # __switch(current_task_cx: &*const TaskContext, next_task_cx: &*const TaskContext)
+    # push TaskContext to current sp and save its address to where a0 points to
+    addi sp, sp, -13*8
+    sd sp, 0(a0)
+    # fill TaskContext with ra & s0-s11
+    sd ra, 0(sp)
+    .set n, 0
+    .rept 12
+        SAVE_SN %n
+        .set n, n + 1
+    .endr
+    # ready for loading TaskContext a1 points to
+    ld sp, 0(a1)
+    # load registers in the TaskContext
+    ld ra, 0(sp)
+    .set n, 0
+    .rept 12
+        LOAD_SN %n
+        .set n, n + 1
+    .endr
+    # pop TaskContext
+    addi sp, sp, 13*8
+    ret
+