[jdk19] RFR: 8288128: S390X: Fix crashes after JDK-8284161 (Virtual Threads)

Tue Jul 5 17:46:00 UTC 2022

This PR adapts the changes to the PPC port from JDK-8286446 and JDK-8288105 to s390 in order to fix the crashes preventing a successful `make images`.

In addition, the following (minor) changes were made to the changesets above:
- Change two instances of `own_abi()->lr` to `own_abi()->return_pc` (frame_s390.cpp:235,247)
- Remove alignment assertion from `frame::setup` (frame_s390.inline.hpp:73)
- Remove original_pc assertion from `frame::patch_pc` (frame_s390.cpp:251)
- Add continuations_enabled guard to `generate_phase1` (stubGenerator_s390.cpp:2937)

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Commit messages:
 - Adapt and apply changes from JDK-8288105 to s390
 - Adapt and apply changes from JDK-8286446 to s390.

Changes: https://git.openjdk.org/jdk19/pull/110/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=110&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288128
  Stats: 193 lines in 7 files changed: 111 ins; 41 del; 41 mod
  Patch: https://git.openjdk.org/jdk19/pull/110.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/110/head:pull/110

PR: https://git.openjdk.org/jdk19/pull/110