RFR: 8280863: Update build README to reflect that MSYS2 is supported [v3]

Fri Jan 28 09:42:11 UTC 2022

On Fri, 28 Jan 2022 09:28:44 GMT, TheShermanTanker <duke at openjdk.java.net> wrote:

>> Ever since [JDK-8257679](https://bugs.openjdk.java.net/browse/JDK-8257679) was merged in 16, MSYS2 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 This PR updates some outdated documentation in building.html and building.md to reflect that change.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.
>
> TheShermanTanker has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update htmlm using pandoc.

Looks good now. Continue by giving the `/integrate` command, I'll sponsor it for you.

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Marked as reviewed by ihse (Reviewer).

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