Official and community supported build platforms for JDK 8 and 9

[Magnus Ihse Bursie]

A recurring theme in the build-dev list is confusion on which platforms 
it is possible to build OpenJDK. Unfortunately, information about this 
has not been easy to gather. It has also not been clear what kind of 
build issues the Build Team will respond to and with what kind of urgency.

To help address this, I've created a publicly available wiki page: 
https://wiki.openjdk.java.net/display/Build/Supported+build+platforms

Support for building on different platforms come in two varieties: 
official support and community support.

Oracle defines a number of official build platforms, with carefully 
specified versions of operating systems, compilers and other build 
tools. If you report a failure to build on any of these platforms, the 
report will be processed with high priority from the build team in 
Oracle. Under normal circumstances, a build on any of these platforms 
will always succeed.

In addition to the official build platforms, OpenJDK can normally be 
built on many more platforms. For these platforms, there is no guarantee 
that the build will succeed. The Oracle build team can help to solve 
some problems encountered, but only on a best-effort basis. In addition 
to the Oracle build team, the OpenJDK community at large is welcome to 
help with making OpenJDK compile on these platforms.

The official and community supported build platforms are listed on the 
wiki page. Note that build support is different for different versions 
of the JDK. We welcome updates from the community to the list of 
community supported platforms. If you have succeeded (or not!) in 
building OpenJDK on a platform that is substantially different from the 
ones already listed, please document your experience in the list.

The list of community supported platforms on the wiki is currently much 
shorter than the number of platforms I expect OpenJDK to build on. Once 
again, please help us by filling in the wiki!

/Magnus