[jdk17u-dev] RFR: 8280863: Update build README to reflect that MSYS2 is supported

[George Adams]

Backport [JDK-8257679](https://bugs.openjdk.java.net/browse/JDK-8257679) as it's a non trivial change

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Commit messages:
 - Backport 6de90ad9800b83c4a5f364c3645603fcb6828d6c

Changes: https://git.openjdk.org/jdk17u-dev/pull/540/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280863
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/540.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/540/head:pull/540

PR: https://git.openjdk.org/jdk17u-dev/pull/540