RFR: 8349457: [leyden] Workaround Leyden problems with -XX:-TieredCompilation

[Aleksey Shipilev]

Currently, supplying `-XX:-TieredCompilation` would break with Leyden workflows, because the archive-backed code cache heap would only be created if `SegmentedCodeCache` is enabled. With default compilation policy, this happens normally when tiered compilation is enabled, but not without it.

There are other minor peculiarities that we need to handle, which are exposed with `runtime/cds` + `-XX:-TieredCompilation` runs.

This prevents us from testing C2-only Leyden configs and studying compilation dynamics without tiered policy getting in the way.

Additional testing:
 - [x] Ad-hoc Leyden experiments now work with `-XX:-TieredCompilation`: the C2-only code is being stored and loaded successfully from the CDS archive
 - [x] `runtime/cds` now passes with `-XX:-TieredCompilation`
 - [ ] `runtime/cds` still passes by default (i.e. with tiered compilation)

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Commit messages:
 - More fixes
 - Fix

Changes: https://git.openjdk.org/leyden/pull/32/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=32&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349457
  Stats: 38 lines in 3 files changed: 29 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/leyden/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/32/head:pull/32

PR: https://git.openjdk.org/leyden/pull/32