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.
This is necessary to support the ESP32-C3, which lacks the A (atomic)
extension and thus requires these 32-bit atomic operations.
With this commit, flashing ./testdata/atomic.go to the ESP32-C3 works
correctly and produces the expected output on the serial console.
