runtime (gc): scan callee-saved registers while marking stack

src/runtime/scheduler_avr.S

@@ -187,3 +187,55 @@ tinygo_switchToScheduler:
 
     // Return into the scheduler, as if tinygo_switchToTask was a regular call.
     ret
+
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Save callee-saved registers.
+    push r29 // Y
+    push r28 // Y
+    push r17
+    push r16
+    push r15
+    push r14
+    push r13
+    push r12
+    push r11
+    push r10
+    push r9
+    push r8
+    push r7
+    push r6
+    push r5
+    push r4
+    push r3
+    push r2
+
+    // Scan the stack.
+    in  r24, 0x3d; SPL
+    in  r25, 0x3e; SPH
+#if __AVR_ARCH__ == 2 || __AVR_ARCH__ == 25
+    rcall tinygo_scanstack
+#else
+    call tinygo_scanstack
+#endif
+
+    // Restore callee-saved registers.
+    pop r2
+    pop r3
+    pop r4
+    pop r5
+    pop r6
+    pop r7
+    pop r8
+    pop r9
+    pop r10
+    pop r11
+    pop r12
+    pop r13
+    pop r14
+    pop r15
+    pop r16
+    pop r17
+    pop r28 // Y
+    pop r29 // Y