runtime: move timeUnit to a single place

The timeUnit is now the same type everywhere. Move it to a single place
and add some documentation to it.

src/runtime/runtime_arm7tdmi.go

@@ -7,8 +7,6 @@ import (
 	"unsafe"
 )
 
-type timeUnit int64
-
 func putchar(c byte) {
 	// dummy, TODO
 }