RFR: 7903206: Support MSYS2 for building jtreg on Windows

[Christian Stein]

7903206: Support MSYS2 for building jtreg on Windows

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Commit messages:
 - Look for `MINGW` in `uname` as well
 - 7903206: Support MSYS2 for building jtreg on Windows

Changes: https://git.openjdk.java.net/jtreg/pull/87/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=87&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903206
  Stats: 16 lines in 3 files changed: 9 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/87.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/87/head:pull/87

PR: https://git.openjdk.java.net/jtreg/pull/87