std: move time implementations to sys (small platforms)

Let's start with the easy ones:
* Motor just reexports its platform library
* The SGX code is just a trivial move
* Trusty, WASM and ZKVM are unsupported, this is very trivial. And we can get
  rid of some `#[path = ...]`s, yay!

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

@@ -1,7 +1,6 @@
 #![allow(unsafe_op_in_unsafe_fn)]
 
 pub mod os;
-pub mod time;
 
 pub use moto_rt::futex;
 

library/std/src/sys/pal/motor/time.rs

@@ -1 +0,0 @@
-pub use moto_rt::time::{Instant, SystemTime, UNIX_EPOCH};

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

@@ -12,7 +12,6 @@ pub mod abi;
 mod libunwind_integration;
 pub mod os;
 pub mod thread_parking;
-pub mod time;
 pub mod waitqueue;
 
 // SAFETY: must be called only once during runtime initialization.

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

@@ -5,7 +5,5 @@
 mod common;
 #[path = "../unsupported/os.rs"]
 pub mod os;
-#[path = "../unsupported/time.rs"]
-pub mod time;
 
 pub use common::*;

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

@@ -1,7 +1,6 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 
 pub mod os;
-pub mod time;
 
 mod common;
 pub use common::*;

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

@@ -18,8 +18,6 @@
 
 #[path = "../unsupported/os.rs"]
 pub mod os;
-#[path = "../unsupported/time.rs"]
-pub mod time;
 
 #[cfg(target_feature = "atomics")]
 #[path = "atomics/futex.rs"]

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

@@ -1,7 +1,6 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
 pub mod os;
-pub mod time;
 
 #[path = "../unsupported/common.rs"]
 mod common;

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

@@ -12,8 +12,6 @@ pub const WORD_SIZE: usize = size_of::<u32>();
 
 pub mod abi;
 pub mod os;
-#[path = "../unsupported/time.rs"]
-pub mod time;
 
 use crate::io as std_io;
 

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

@@ -1,4 +1,19 @@
 cfg_select! {
+    target_os = "motor" => {
+        use moto_rt::time as imp;
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
+        mod sgx;
+        use sgx as imp;
+    }
+    target_os = "xous" => {
+        mod xous;
+        use xous as imp;
+    }
+    _ => {
+        mod unsupported;
+        use unsupported as imp;
+    }
 }
 
 pub use imp::{Instant, SystemTime, UNIX_EPOCH};

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

@@ -1,4 +1,4 @@
-use super::abi::usercalls;
+use crate::sys::pal::abi::usercalls;
 use crate::time::Duration;
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]