machine: make gosched available to machine package

src/machine/machine_nxpmk66f18_uart.go

@@ -59,9 +59,6 @@ var (
 	ErrNotConfigured  = errors.New("device has not been configured")
 )
 
-//go:linkname gosched runtime.Gosched
-func gosched()
-
 // PutcharUART writes a byte to the UART synchronously, without using interrupts
 // or calling the scheduler
 func PutcharUART(u *UART, c byte) {

src/machine/runtime.go

@@ -0,0 +1,13 @@
+package machine
+
+import (
+	_ "unsafe"
+)
+
+//
+// This file provides access to runtime package that would not otherwise
+// be permitted due to linker dependencies.
+//
+
+//go:linkname gosched runtime.Gosched
+func gosched()