RFR: 8252191: Update to gcc 10.2 on Linux

[Kevin Rushforth]

This patch updates the compiler to gcc 10.2 on Linux, in order to match JDK 16 -- see
[JDK-8253616](https://bugs.openjdk.java.net/browse/JDK-8253616).

I ran a full build and test, including media and WebKit.

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Commit messages:
 - 8252191: Update to gcc 10.2 on Linux

Changes: https://git.openjdk.java.net/jfx/pull/311/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=311&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252191
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/311.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/311/head:pull/311

PR: https://git.openjdk.java.net/jfx/pull/311