RFR: 8342409: [s390x] C1 unwind_handler fails to unlock synchronized methods with LM_MONITOR

[Amit Kumar]

Make sure LIR_Assembler::emit_unwind_handler() jumps to the slow path directly for unlocking a synchronized method if LM_MONITOR is used.
On the fast paths assertions are added that the mode is actually handled.

Testing: Tier1 test for fastdebug vm showed no regression.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/21557/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21557&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342409
  Stats: 9 lines in 2 files changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21557.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21557/head:pull/21557

PR: https://git.openjdk.org/jdk/pull/21557