RFR: 8341862: PPC64: C1 unwind_handler fails to unlock synchronized methods with LM_MONITOR

[Richard Reingruber]

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.

The change passed our CI testing:
Tier 1-4 of hotspot and jdk. All of Langtools and jaxp. Renaissance Suite and SAP specific tests.
Testing was done on the main platforms and also on Linux/PPC64le and AIX.

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Commit messages:
 - C1: fix unlock in unwind handler for LM_MONITOR

Changes: https://git.openjdk.org/jdk/pull/21497/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21497&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341862
  Stats: 9 lines in 2 files changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21497.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21497/head:pull/21497

PR: https://git.openjdk.org/jdk/pull/21497