RFR: 8356251: Need minor cleanup for interp_only_mode

[Serguei Spitsyn]

This is a minor cleanup for the JVMTI `interp_only_mode` implementation which includes the following changes:
 - The `interp_only_mode` in `JavaThread` is represented with a counter which is increment and decremented. This is confusing because this value should only take values `0` and `1`. Asserts are placed to make sure it is never going out of bounds. The `interp_only_mode` in a `JavaThread` is checked by the interpreter chunks which expect it to be an `integer`. This cleanup has no intention to make it a boolean.
 - The function `JvmtiThreadState::process_pending_interp_only()` does a sync on the `JvmtiThreadState_lock` which is not really needed and is being removed. It is called in a `VTMS` transition and so, can not clash with the `SetEventNotificationMode` because it sets a `JvmtiVTMSTransitionDisabler`.
 
 Testing:
  - TBD: Mach5 tiers 1-6

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Commit messages:
 - 8356251: Need minor cleanup for interp_only_mode

Changes: https://git.openjdk.org/jdk/pull/25060/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356251
  Stats: 11 lines in 3 files changed: 1 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25060.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25060/head:pull/25060

PR: https://git.openjdk.org/jdk/pull/25060