all: move bootstrapping IR to Go runtime

This has the benefit of not requiring a 'runtime' IR file, so that
complete relocatable files can be built without requiring input IR.
This makes the compiler a lot easier to use without the Makefile.

Code size is not affected.

src/runtime/runtime_nrf.c

@@ -29,10 +29,6 @@ void RTC0_IRQHandler() {
 	rtc_wakeup = true;
 }
 
-void _start() {
-	main();
-}
-
 __attribute__((weak))
 void __aeabi_unwind_cpp_pr0() {
 	// dummy, not actually used