machine/stm32: add support for stm32f103xx UART

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/runtime/runtime_stm32.go

@@ -2,14 +2,8 @@
 
 package runtime
 
-import (
-	"device/arm"
-)
-
 type timeUnit int64
 
-const tickMicros = 1 // TODO
-
 //go:export Reset_Handler
 func main() {
 	preinit()
@@ -17,18 +11,3 @@ func main() {
 	mainWrapper()
 	abort()
 }
-
-func putchar(c byte) {
-	// TODO
-}
-
-func sleepTicks(d timeUnit) {
-	// TODO: use a real timer here
-	for i := 0; i < int(d/535); i++ {
-		arm.Asm("")
-	}
-}
-
-func ticks() timeUnit {
-	return 0 // TODO
-}