Ensure SystemUTCEpochNanoseconds never throws

Times outside of a range of about half a million years cannot be
represented by Temporal.Instant. Clamp the system time to that range so
that the operation is guaranteed not to throw, which is editorially the
cleanest solution. If your clock is 300,000 years off then Temporal is not
the biggest problem you will have, so this is not expected to come up in
practice.

See: #1424

polyfill/lib/ecmascript.mjs

@@ -4315,7 +4315,7 @@ export const ES = ObjectAssign({}, ES2020, {
       const ms = Date.now();
       const result = bigInt(ms).multiply(1e6).plus(ns);
       ns = ms % 1e6;
-      return result;
+      return bigInt.min(NS_MAX, bigInt.max(NS_MIN, result));
     };
   })(),
   SystemTimeZone: () => {

spec/temporal.html

@@ -192,20 +192,26 @@ <h1>SystemTimeZone ( )</h1>
       <h1>SystemUTCEpochNanoseconds ( )</h1>
       <emu-alg>
         1. Let _ns_ be the approximate current UTC date and time, in nanoseconds since the epoch.
-        1. Assert: ! ValidateInstant(_ns_) is *true*.
+        1. Set _ns_ to the result of clamping _ns_ between −8.64 × 10<sup>21</sup> and 8.64 × 10<sup>21</sup>.
         1. Return _ns_.
       </emu-alg>
       <emu-note>
         <p>The precision of the result depends on the host. In particular, web browsers artificially
         limit it to prevent abuse of security flaws (e.g., Spectre) and to avoid certain methods of
         fingerprinting.</p>
       </emu-note>
+      <emu-note>
+        <p>
+          Step 2 is necessary if the system clock is set to a time outside the range that Temporal.Instant can represent.
+          This is not expected to affect implementations in practice.
+        </p>
+      </emu-note>
     </emu-clause>
 
     <emu-clause id="sec-temporal-systeminstant" aoid="SystemInstant">
       <h1>SystemInstant ( )</h1>
       <emu-alg>
-        1. Let _ns_ be ? SystemUTCEpochNanoseconds().
+        1. Let _ns_ be ! SystemUTCEpochNanoseconds().
         1. Return ? CreateTemporalInstant(_ns_).
       </emu-alg>
     </emu-clause>
@@ -231,7 +237,7 @@ <h1>SystemZonedDateTime ( [ _temporalTimeZoneLike_ [ , _calendarLike_ ] ] )</h1>
         1. Else,
           1. Let _timeZone_ be ? ToTemporalTimeZone(_temporalTimeZoneLike_).
         1. Let _calendar_ be ? ToTemporalCalendar(_calendarLike_).
-        1. Let _ns_ be ? SystemUTCEpochNanoseconds().
+        1. Let _ns_ be ! SystemUTCEpochNanoseconds().
         1. Return ? CreateTemporalZonedDateTime(_ns_, _timeZone_, _calendar_).
       </emu-alg>
     </emu-clause>