pal/sgx: Implement SystemTime::{MIN, MAX}

This commit implements `SystemTime::MIN` and `SystemTime::MAX` for sgx.

The implementation uses a `Duration` to store the Unix time, thereby
implying `Duration::ZERO` and `Duration::MAX` as the limits.

Author: cvengler

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

@@ -28,6 +28,12 @@ impl Instant {
 }
 
 impl SystemTime {
+    #[unstable(feature = "time_systemtime_limits", issue = "none")]
+    pub const MAX: SystemTime = SystemTime(Duration::MAX);
+
+    #[unstable(feature = "time_systemtime_limits", issue = "none")]
+    pub const MIN: SystemTime = SystemTime(Duration::ZERO);
+
     pub fn now() -> SystemTime {
         SystemTime(usercalls::insecure_time())
     }