[13u] RFR (S) Backport JDK-8191521: handle long relative path specified in -Xbootclasspath/a on windows

[Schmelter, Ralf]

I'd like to backport the change to 13u.

Original bug:
https://bugs.openjdk.java.net/browse/JDK-8191521

Original change:
https://hg.openjdk.java.net/jdk/jdk/rev/ed5e399d967d

11u backport:
http://cr.openjdk.java.net/~rschmelter/webrevs/8191521/webrev.jdku13.01/

The patch had to be modified since it also adjusted the os:: same_files() method, which is not present in JDK13.
Otherwise it applied cleanly.

Best regards,
Ralf