RFR: 8242490: Upgrade to gcc 9.2 on Linux

[Kevin Rushforth]

This is a compiler upgrade on Linux from the current gcc 8.3 compiler to gcc 9.2. This will match a recent upgrade done
for JDK 15 -- see [JDK-8241721](https://bugs.openjdk.java.net/browse/JDK-8241721).

On a related note, using the gcc 9 compiler produces many (harmless) warnings that will be addressed by
[JDK-8241476](https://bugs.openjdk.java.net/browse/JDK-8241476) -- see PR #150.

I have run a full build and test using this new compiler.

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Commit messages:
 - 8242490: Upgrade to gcc 9.2 on Linux

Changes: https://git.openjdk.java.net/jfx/pull/178/files
 Webrev: https://webrevs.openjdk.java.net/jfx/178/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242490
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/178.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/178/head:pull/178

PR: https://git.openjdk.java.net/jfx/pull/178