RFR: 8241721: Change to GCC 9.2 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 9.2.

JBS: https://bugs.openjdk.java.net/browse/JDK-8241721 <https://bugs.openjdk.java.net/browse/JDK-8241721>
webrev: http://cr.openjdk.java.net/~mikael/webrevs/8241721/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8241721/webrev.00/open/webrev/>

Cheers,
Mikael