mx: change in output directory

[Doug Simon]

I recently made a change to the location where mx puts certain generated artifacts such as class files and annotation processor generated sources:

https://bitbucket.org/allr/mx/commits/9a21cebf621ad46796d08a15d6487bda1060b5aa

For suites that update their mx version dependency to 5.5.6 or later (e.g., [1]), a top level mxbuild/ directory will appear. You should add this to the .hgignore file for the suite (e.g., [2]).

-Doug

[1] http://hg.openjdk.java.net/graal/graal-jvmci-8/rev/452ef008f3c3#l8.6
[2] http://hg.openjdk.java.net/graal/graal-jvmci-8/rev/fcaab0ed7782