rumtime: implement __sync libcalls as critical sections

This change implements __sync atomic polyfill libcalls by disabling interrupts.
This was previously done in a limited capacity on some targets, but this change uses a go:generate to emit all of the calls on all microcontroller targets.

src/runtime/runtime.go

@@ -4,6 +4,8 @@ import (
 	"unsafe"
 )
 
+//go:generate go run ../../tools/gen-critical-atomics -out ./atomics_critical.go
+
 const Compiler = "tinygo"
 
 // The compiler will fill this with calls to the initialization function of each