JDK8 no longer required to develop Graal

[Doug Simon]

Developing Graal has always required a JDK8 to be present in either JAVA_HOME or EXTRA_JAVA_HOMES. Please see http://mail.openjdk.java.net/pipermail/graal-dev/2018-June/005413.html for background on this.

I've recently pushed changes to mx[1] and Graal[2] that removes this requirement. That is, you should now be able to develop Graal with only a JDK11 in JAVA_HOME. I have updated https://github.com/oracle/graal/blob/master/compiler/README.md#building-graal to reflect this.

If you run into any problems developing Graal with only a JDK 11, please open an issue at https://github.com/oracle/graal/issues or send a message to graal-dev at openjdk.java.net.

-Doug

[1] https://github.com/graalvm/mx/commit/c9f873aa979b2a724e7e213fd1e0d4ce7131b87f
[2] https://github.com/oracle/graal/commit/a4070ec3d1b16d49f42694b3f0a46a39010b445c