What does this PR do?:
Reorganizes JVM-specific C++ source into dedicated subdirectories and introduces a new JVMThread class as a JVM-agnostic thread bridge.

• hotspot/ — vmStructs.cpp/h/inline.h moved here; HotSpot-specific thread initialization and execution state logic consolidated into VMThread methods
• j9/ — j9Ext, j9WallClock moved here
• jvmThread.h/cpp (new) — JVM-agnostic JVMThread class that wraps both HotSpot's TLS-based thread lookup and J9's j9thread_self() under a single interface; owns the pthread key and the Java tid field ID previously scattered across VMStructs

Why
Previously VMStructs::initThreadBridge() handled both HotSpot and J9 initialization paths in a single monolithic function. The TLS key, java thread ID field, and thread execution state logic were split across VMStructs, threadState.inline.h, and call sites. This made it difficult to conditionally compile or reason about JVM-specific behavior.

Key changes

• VMStructs::initThreadBridge() removed — replaced by JVMThread::initialize() (JVM-agnostic, called unconditionally in VM::ready()) + VMThread::initialize() (HotSpot-only)
• VMThread::getExecutionMode() / getOSThreadState() — thread state→execution mode conversion moved from threadState.inline.h into VMThread methods; threadState.inline.h now simply delegates to them
• wallClock.cpp signal handler simplified — ~15 lines of inline thread state inspection replaced by getOSThreadState() / getThreadExecutionMode()
• _tls_index / _tid removed from VMStructs — now owned by JVMThread
• Dead code removed — j9StackTraces.h, javaApi.h (empty), stackFrame_{arm,i386,loongarch64,ppc64,riscv64}.cpp (unsupported architectures), VM::_libjava

Motivation:

• This is part one of PROF-13889 that focuses on providing an abstraction on Thread - to avoid PR growing to large. Followup PRs will address other areas, especially, in stack walking.

• In addition, this PR removes some of dead code and files that related to the platforms that we don't support.

Additional Notes:
How to test the change?:

• Regular CI tests
• Stress tests

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: PROF-13889

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