Add printc and exit builtin functions

tools/builtin.s

@@ -1,6 +1,12 @@
 .global printi
 .type printi, @function
 
+.global printc
+.type printc, @function
+
+.global exit
+.type exit, @function
+
 .global _start
 .type _start, @function
 
@@ -53,8 +59,35 @@ printi:
     addi sp, sp, 16
     ret
 
-_start:
-    call main
+printc:
+    addi sp, sp, -12
+    sw s0, 0(sp)
+    sw ra, 4(sp)
+    addi s0, sp, 12
+
+    addi t1, zero, '\n'
+    sb t1, -1(s0)
+
+    lw t0, 0(s0)
+    sb t0, -2(s0)
+
+    addi a0, zero, 1
+    addi a1, s0, -2
+    addi a2, zero, 2
+    addi a7, zero, 64
+    ecall
+
+    lw s0, 0(sp)
+    lw ra, 4(sp)
+    addi sp, sp, 12
+    ret
+
+exit:
     addi a0, zero, 0
     addi a7, zero, 93
     ecall
+    # ret
+
+_start:
+    call main
+    call exit