RFR(XS): 8220164: Fix build instructions for AIX

[Volker Simonis]

Hi,

can I please have a review for the following trivial change which
updates the AIX/XLC information in the the building.{md, html} files:

https://bugs.openjdk.java.net/browse/JDK-8220164
http://cr.openjdk.java.net/~simonis/webrevs/2019/8220164/

The build instructions for AIX in doc/building.{md, html} still
reference our old AIX build results page which isn't active since
quite some time and refers to old OS/compiler versions.

Remove the old information and link to the OpenJDK build Wiki instead
(https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms)
which should always have the current information.

Thank you and best regards,
Volker