RFR: 8257679: Improved unix compatibility layer in Windows build (winenv) (Update building.md and building.html to reflect change)

Thu Jan 27 08:59:44 UTC 2022

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 This PR updates some outdated documentation in building.html and building.md to reflect that change.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

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Commit messages:
 - Update documentation to mention MSys 2 support

Changes: https://git.openjdk.java.net/jdk/pull/7242/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7242&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257679
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 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