Clock.systemUTC() should return a constant

The java.time.Clock::systemUTC() method currently creates an instance every time it is called. It should return a private constant instead, as the returned instance is immutable and could be shared.

The spec does not constrain this method - "The returned implementation is immutable, thread-safe and {@code Serializable}. It is equivalent to {@code system(ZoneOffset.UTC)}." - which would still be satisfied by returning a constant instead of a new instance.