RFR: 8276746: Add section on reproducible builds in building.md

[Severin Gehwolf]

On Fri, 5 Nov 2021 16:30:26 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Reproducible builds are all the vogue. The JDK has been making great strides in getting there, but still has some way to go. However, to get as close as possible, some special configuration is needed. 
> 
> This has been "tribal knowledge" of a few persons in the build team. It needs to be properly documented to help other users wanting to create reproducible builds of the JDK.

LGTM. Thanks for documenting it!

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Marked as reviewed by sgehwolf (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6279