RFR: 1892: Handle jfx cpu versions

[Erik Joelsson]

The JdkVersion parser needs to handle versions on the form "jfx-cpu" and "jfxXXu-cpu". I chose to just accept any prefix string consisting of lower case letters.

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Commit messages:
 - Limit *-cpu to just lower case letters as prefix
 - Accept any prefix on modern version strings
 - SKARA-1892

Changes: https://git.openjdk.org/skara/pull/1509/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1509&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1892
  Stats: 15 lines in 2 files changed: 10 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1509.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1509/head:pull/1509

PR: https://git.openjdk.org/skara/pull/1509