RFR: JDK-8200867 Remove references to "jdk 9" in build system

[Magnus Ihse Bursie]

There were some lingering references to jdk 9 in the build 
documentation. Remove this (and do not replace with new version numbers, 
due to our accelerated versioning scheme).

 From the bug report:

$ bash ./configure --help | head -1
`configure' configures OpenJDK jdk9 to adapt to many kinds of systems.

---

$ grep -i 'jdk.9' doc/building.md
This table lists the OS versions used by Oracle when building JDK 9. Such
OpenJDK 9, the only supported such layer is Cygwin. (Msys is no longer
OpenJDK 9 includes patches that should allow gcc 6 to compile, but this 
should
should be a JDK of major version *N-1*, so for building JDK 9 a JDK 8 
would be
version, since JDK 8 GA might not be able to build JDK 9 on all platforms.
building JDK 9.)
make targets. All source code in JDK 9 is organized so it belongs to a 
module,

Bug: https://bugs.openjdk.java.net/browse/JDK-8200867
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8200867-remove-jdk9-references/webrev.01

/Magnus