RFR: 8367982: Unify ObjectSynchronizer and LightweightSynchronizer [v2]

[Fredrik Bredberg]

> This is the last PR in a series of PRs (see: [JDK-8344261](https://bugs.openjdk.org/browse/JDK-8344261)) to obsolete the LockingMode flag and related code.
> 
> The main focus is to to unify `ObjectSynchronizer` and `LightweightSynchronizer`.
> There used to be a number of "dispatch functions" to redirect calls depending on the setting of the `LockingMode` flag.
> Since we now only have lightweight locking, there is no longer any need for those dispatch functions, so I removed them.
> To remove the dispatch functions I renamed the corresponding lightweight functions and call them directly.
> This ultimately led me to remove "lightweight" from the function names and go back to "fast" instead, just to avoid having some with, and some without the "lightweight" part of the name.
> 
> This PR also include a small simplification of `ObjectSynchronizer::FastHashCode`.
> 
> Tested tier1-7 (on supported platforms) without seeing any problems that can be traced to this code change.
> All other platforms (`arm`, `ppc`, `riscv`, `s390`) has been sanity checked using QEMU.

Fredrik Bredberg has updated the pull request incrementally with one additional commit since the last revision:

  Update after review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27915/files
  - new: https://git.openjdk.org/jdk/pull/27915/files/993d6137..6e331721

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27915&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27915&range=00-01

  Stats: 43 lines in 22 files changed: 4 ins; 0 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/27915.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27915/head:pull/27915

PR: https://git.openjdk.org/jdk/pull/27915