std: move time implementations to sys (VEX)

Now that the `unsupported` module exists, we can use it for VEX. VEX actually
supports `Instant` though, so the implementation-select needs to combine that
with the `unsupported` module.

library/std/src/sys/pal/vexos/mod.rs

@@ -1,5 +1,4 @@
 pub mod os;
-pub mod time;
 
 #[expect(dead_code)]
 #[path = "../unsupported/common.rs"]

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

@@ -6,6 +6,16 @@ cfg_select! {
         mod sgx;
         use sgx as imp;
     }
+    target_os = "vexos" => {
+        mod vexos;
+        #[expect(unused)]
+        mod unsupported;
+
+        mod imp {
+            pub use super::vexos::Instant;
+            pub use super::unsupported::{SystemTime, UNIX_EPOCH};
+        }
+    }
     target_os = "xous" => {
         mod xous;
         use xous as imp;

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

@@ -1,10 +1,5 @@
 use crate::time::Duration;
 
-#[expect(dead_code)]
-#[path = "../unsupported/time.rs"]
-mod unsupported_time;
-pub use unsupported_time::{SystemTime, UNIX_EPOCH};
-
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 pub struct Instant(Duration);