RFR: JDK-8179892 Update build documentation for JDK 9

[Magnus Ihse Bursie]

Also known as: "!!!!!!!!!!!!!!! THIS IS A MAJOR RE-WRITE of this 
document. !!!!!!!!!!!!!" :-)

The build documentation has been in need of an overhaul for a very long 
time. The document is not very well structured, and contains not only 
outdated but also irrelevant information, all the while valuable 
information is missing.

For some time now, I've worked as a background project to rewrite the 
build documentation to address these shortcomings. Now it's finally done.

Here is the new documentation in html format: 
http://cr.openjdk.java.net/~ihse/demo-new-build-readme/common/doc/building.html

And here it is in markdown: 
http://cr.openjdk.java.net/~ihse/JDK-8179892-update-build-readme/webrev.01/raw_files/new/common/doc/building.md

I've tried to make the document as readable as possible in raw markdown, 
hoping it can be of assistance for command-line users.

Bug: https://bugs.openjdk.java.net/browse/JDK-8179892
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8179892-update-build-readme/webrev.01

/Magnus