RFR: 8253616: Change to GCC 10.2 for building on Linux at Oracle

[Mikael Vidstedt]

Please review this change which updates the defaults in the linux devkit creator, the doc/building.md file, and the JIB
(Oracle) configuration to gcc 10.2.

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Commit messages:
 - 8253616: Change to GCC 10.2 for building on Linux at Oracle

Changes: https://git.openjdk.java.net/jdk/pull/349/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=349&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253616
  Stats: 33 lines in 4 files changed: 20 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/349.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/349/head:pull/349

PR: https://git.openjdk.java.net/jdk/pull/349