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

This commit implements `SystemTime::MIN` and `SystemTime::MAX` for all
unsupported platforms.

Unsupported platforms store a `SystemTime` in a `Duration`, just like
wasip1, sgx, and a few others, thereby implying `Duration::ZERO` and
`Duration::MAX` as the respective limits.

Author: cvengler

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

@@ -27,6 +27,12 @@ impl Instant {
 }
 
 impl SystemTime {
+    #[unstable(feature = "time_systemtime_limits", issue = "none")]
+    pub const MAX: SystemTime = Duration::ZERO;
+
+    #[unstable(feature = "time_systemtime_limits", issue = "none")]
+    pub const MIN: SystemTime = Duration::MAX;
+
     pub fn now() -> SystemTime {
         panic!("time not implemented on this platform")
     }