std: move time implementations to sys (preparation)

library/std/src/sys/mod.rs

@@ -26,6 +26,7 @@ pub mod stdio;
 pub mod sync;
 pub mod thread;
 pub mod thread_local;
+pub mod time;
 
 // FIXME(117276): remove this, move feature implementations into individual
 //                submodules.

library/std/src/sys/time/mod.rs

@@ -0,0 +1,4 @@
+cfg_select! {
+}
+
+pub use imp::{Instant, SystemTime, UNIX_EPOCH};