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

[TheShermanTanker]

> 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 This PR updates some outdated documentation in building.html and building.md to reflect that change.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.

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

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7242/files
  - new: https://git.openjdk.java.net/jdk/pull/7242/files/ec5dd921..e0a4896e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7242&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7242&range=00-01

  Stats: 1115 lines in 77 files changed: 438 ins; 379 del; 298 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7242/head:pull/7242

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