RFR: JDK-8200375: Change to GCC 7.3.0 for building Linux at Oracle

[Erik Joelsson]

This patch changes the default devkit used to produce builds for Linux 
x64 at Oracle. The new devkit is based on GCC 7.3.0.

Webrev: http://cr.openjdk.java.net/~erikj/8200375/webrev.01/

Bug: https://bugs.openjdk.java.net/browse/JDK-8200375

/Erik