avr: fix race condition in stack write

If an interrupt happens between the writes to SPL and SPH, the stack
pointer is inconsistent and terrible things will happen. Therefore,
disable interrupts while updating the stack pointer.

Interrupts are restored _before_ the write to SPH. This is safe, because
interrupts are re-enabled with a one cycle delay. The avr-gcc and Clang
compilers do the same thing when they need to update the stack pointer.

It's almost impossible to test for this bug, but it should make firmware
just a little bit more reliable.

src/internal/task/task_stack_avr.S

@@ -72,7 +72,10 @@ tinygo_swapTask:
     std Y+1, r3
 
     // Switch to the new stack pointer.
+    in   r0, 0x3f ; SREG
+    cli
     out  0x3d, r24; SPL
+    out  0x3f, r0 ; SREG, restore interrupts (after the next instruction)
     out  0x3e, r25; SPH
 
     // Load saved register from the new stack.