RFR: 8280863: Update build README to reflect that MSYS2 is supported [v2]

[Magnus Ihse Bursie]

On Fri, 28 Jan 2022 08:49:56 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 This PR updates some outdated documentation in building.html and building.md to reflect that change.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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Clean up documentation and MSys 2 -> MSYS2
>  - Update documentation to mention MSys 2 support

I'm not quite sure of the best way to help you with the html part. I've pushed a fix to https://github.com/magicus/jdk/tree/pull/7242, it is available as commit https://github.com/openjdk/jdk/commit/85024eb1e84425ead40fcc2278ca085ca8fbb5df. Try merging that into your branch.

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