Remove use of CGo in the runtime

CGo depends on syscall, which (in the standard library) depends on sync,
which depends on the runtime. There are also other import cycles. To be
able to use the syscall package from upstream, stop using CGo.

src/runtime/runtime_nrf.go

@@ -2,14 +2,13 @@
 
 package runtime
 
-// #include "runtime_nrf.h"
-import "C"
-
 import (
 	"device/arm"
 	"device/nrf"
 )
 
+func _Cfunc_rtc_sleep(ticks uint32)
+
 const Microsecond = 1
 
 func init() {
@@ -51,7 +50,7 @@ func sleep(d Duration) {
 	for ticks64 != 0 {
 		monotime()                          // update timestamp
 		ticks := uint32(ticks64) & 0x7fffff // 23 bits (to be on the safe side)
-		C.rtc_sleep(C.uint32_t(ticks))      // TODO: not accurate (must be d / 30.5175...)
+		_Cfunc_rtc_sleep(ticks)             // TODO: not accurate (must be d / 30.5175...)
 		ticks64 -= Duration(ticks)
 	}
 }