gcc: Use register r31 to call routines instead of r10

gcc/call.h

@@ -4,13 +4,13 @@
 #define STR(x) STR2(x)
 
 #define _call2(c, jt_addr)           \
-    asm("li a0,ret_" #c "\n"         \
+    asm("li t6,ret_" #c "\n"         \
         "addi sp,sp,-4\n"            \
-        "sw a0,0(sp)\n"              \
-        "li a0,[" STR(jt_addr) "]\n" \
-        "jr a0\n"                    \
+        "sw t6,0(sp)\n"              \
+        "li t6,[" STR(jt_addr) "]\n" \
+        "jr t6\n"                    \
         "ret_" #c ":"                \
-        ::: "a0"                     \
+        ::: "t6"                     \
     );
 #define _call(c, jt_addr) _call2(c, jt_addr)
 #define call(jt_addr) _call(__COUNTER__, jt_addr)