RFR: 7903206: Support MSYS2 for building jtreg on Windows [v2]

[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:

  Fix typo and print value of `CYGWIN_OR_MSYS`

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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/3919f808..1dc45a92

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=87&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=87&range=00-01

  Stats: 2 lines in 1 file changed: 1 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