RFR: JDK-8175311: Jib sets bad JT_JAVA on linux aarch64

[Erik Joelsson]

Open part of Oracle specific change for the Jib configuration. Moves the 
definition of the boot_jdk_platform to common to make it possible to 
override from the closed file.

Bug: https://bugs.openjdk.java.net/browse/JDK-8175311
Webrev: http://cr.openjdk.java.net/~erikj/8175311/webrev.01/

/Erik