JDK 9 required/recommended for building Graal

[Doug Simon]

As of https://github.com/oracle/graal/commit/6b9b1e38299a73d05dd23c41c667fbb3966c61e7, to *build* Graal sources with mx, you need* to have JDK 9 or later specified in EXTRA_JAVA_HOMES (unless JAVA_HOME is already pointing to a JDK 9 or later in which case no need to do anything). Any released JDK 9 or later is sufficient.


* You can actually omit configuring a JDK 9 or later in EXTRA_JAVA_HOMES if you're willing to deal with this interactive dialog when doing something like `mx build`:
Please select a JDK for building projects with compliance 9: (version >=9)
[1] Java 10 (10) from /Library/Java/JavaVirtualMachines/jdk-10.jdk/Contents/Home
[2] Java 10 (10) from /Library/Java/JavaVirtualMachines/openjdk-10.jdk/Contents/Home
[3] Java 9.0.4 (9) from /Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents/Home
[4] Java 9.0.1 (9) from /Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home
[5] Java 9.0.1 (9) from /Library/Java/JavaVirtualMachines/openjdk-9.0.1.jdk/Contents/Home
[6] Java 9 (9) from /Library/Java/JavaVirtualMachines/labsjdk-9+181
[7] <other>
[8] Cancel (some projects will be removed which may result in errors)
Enter number of selection:

-Doug