RFR: 7903206: Support MSYS2 for building jtreg on Windows [v3]

[Christian Stein]

> Support MSYS2 for building jtreg on Windows.

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Mix path to JDKHOME

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Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/87/files
  - new: https://git.openjdk.java.net/jtreg/pull/87/files/1dc45a92..68ea5432

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=87&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=87&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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