RFR: 8280863: Update build README to reflect that MSYS2 is supported

Fri Jan 28 08:49:58 UTC 2022

On Thu, 27 Jan 2022 08:49:08 GMT, TheShermanTanker <duke at openjdk.java.net> wrote:

> Ever since [JDK-8257679](https://bugs.openjdk.java.net/browse/JDK-8257679) was merged in 16, MSys 2 became capable of building a functional JDK on Windows. This PR updates some outdated documentation in building.html and building.md to reflect that change.
> 
> I appreciate any reviews, and would like to know how I might be able to improve the wording of both updated docs, if possible.

Updated MSYS2 name and prettify building.md (html left out as requested)

@magicus Thank you!

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PR: https://git.openjdk.java.net/jdk/pull/7242