RFR: 8344560: Add system property for patched runtime

[Severin Gehwolf]

Please review this simple patch which adds a new external system property `jdk.patched` when the runtime has been patched with the `--patch-module` switch. This is useful for two reasons: 1) it allows one to determine at runtime whether or not `--patch-module` has been used (by querying the said property) 2) allows tools, such as `jlink` doing the same without exposing more of the internal properties the JVM sets on initialization.

CSR is forthcoming.

Testing:

- [x] GHA
- [x] New jtreg test 

Thoughts?

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Commit messages:
 - 8344560: Add system property for patched runtime

Changes: https://git.openjdk.org/jdk/pull/22277/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22277&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344560
  Stats: 112 lines in 3 files changed: 111 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22277.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22277/head:pull/22277

PR: https://git.openjdk.org/jdk/pull/22277