RFR: 8162439: Runtime.Version.parse needs fast-path for major versions

[Claes Redestad]

Hi,

please review this patch to address a startup regression due to use of 
Runtime.Version.parse("8") etc in JarFile, as introduced by JDK-8150680. 
This solution introduce a fast-path in case of what appears to be a 
single number is sent to Runtime.Version.parse to avoid initializing 
Runtime.VersionBuilder:

Webrev: http://cr.openjdk.java.net/~redestad/8162439/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8162439

Passes all existing tests.

Thanks!

/Claes