RFR: 8370207: Test sun/misc/SunMiscSignalTest.java crashes after JDK-8369631

[David Holmes]

I missed that the `UserHandler` can still be used even if `-Xrs` is specified, so we have to check for `!ReduceSignalUsage` as per Kim's initial suggestion in JDK-8369631.

Testing
- tiers 1-4

Thanks

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Commit messages:
 - 8370207: Test sun/misc/SunMiscSignalTest.java crashes after JDK-8369631

Changes: https://git.openjdk.org/jdk/pull/27888/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27888&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370207
  Stats: 5 lines in 1 file changed: 1 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27888.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27888/head:pull/27888

PR: https://git.openjdk.org/jdk/pull/27888