compiler: work around AVR atomics bugs

The AVR backend has several critical atomics bugs.
This change invokes libcalls for all atomic operations on AVR.
Now `testdata/atomic.go` compiles and runs correctly.

src/runtime/arch_avr.go

@@ -1,7 +1,10 @@
+//go:build avr
 // +build avr
 
 package runtime
 
+import "runtime/interrupt"
+
 const GOARCH = "arm" // avr pretends to be arm
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -16,3 +19,19 @@ func align(ptr uintptr) uintptr {
 func getCurrentStackPointer() uintptr {
 	return uintptr(stacksave())
 }
+
+// The safest thing to do here would just be to disable interrupts for
+// procPin/procUnpin. Note that a global variable is safe in this case, as any
+// access to procPinnedMask will happen with interrupts disabled.
+
+var procPinnedMask interrupt.State
+
+//go:linkname procPin sync/atomic.runtime_procPin
+func procPin() {
+	procPinnedMask = interrupt.Disable()
+}
+
+//go:linkname procUnpin sync/atomic.runtime_procUnpin
+func procUnpin() {
+	interrupt.Restore(procPinnedMask)
+}