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

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

It is similar to wasip1, wasip2, and sgx in the sense of once again
using a `Duration` to store the value.

Author: cvengler

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

@@ -35,6 +35,12 @@ 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 {
         let result = blocking_scalar(systime_server(), GetUtcTimeMs.into())
             .expect("failed to request utc time in ms");