Remove obsolete and redundant VM assembly

[EgorBo]

Remove unused VM assembly helpers and their leftover declarations, imports, and state.

Generated by GitHub Copilot.

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[Copilot]

🔵 Needs a closer look

It modifies low-level, platform-specific CoreCLR assembly and write-barrier patching, which has high regression risk across rarely-built configurations.

Pull request overview

This PR removes obsolete CoreCLR VM assembly and associated support code across multiple architectures, while simplifying the AMD64 write-barrier placeholder by reserving an explicit fixed-size buffer used by WriteBarrierManager patching.

Changes:

• Replaces the duplicated AMD64 JIT_WriteBarrier placeholder implementation with a fixed 256-byte buffer (new JIT_WRITE_BARRIER_BUFFER_SIZE constant).
• Removes unused write-barrier location globals, unused x86/AMD64 helper routines, and stale comments tied to deleted stubs.
• Deletes dead/disabled fixup and stack-probing stubs (e.g., FEATURE_PREJIT LoongArch64 stubs, ARM stack-probe emitter chain), plus unused imports.

File summaries

File	Description
src/coreclr/vm/virtualcallstub.cpp	Removes obsolete debugger-only extern tied to deleted patch label.
src/coreclr/vm/threads.cpp	Cleans up write-barrier location globals and initialization paths (notably x86 vs non-x86).
src/coreclr/vm/loongarch64/asmhelpers.S	Deletes FEATURE_PREJIT NGEN-era fixup stubs and patch labels.
src/coreclr/vm/i386/jithelp.S	Removes unused x86 write-barrier location global storage.
src/coreclr/vm/i386/jithelp.asm	Removes unused x86 write-barrier location global storage (MASM).
src/coreclr/vm/i386/cgenx86.cpp	Updates comment to reflect remaining live worker paths.
src/coreclr/vm/i386/asmhelpers.S	Removes Unix-side x86 FPU helpers now treated as Windows-only.
src/coreclr/vm/frames.cpp	Updates debugger trace comment to match remaining behavior.
src/coreclr/vm/excep.cpp	Makes the x86 FPU-state holder clearly Windows-only by moving #ifndef TARGET_UNIX.
src/coreclr/vm/cgensys.h	Removes obsolete VirtualMethodFixup* declarations.
src/coreclr/vm/arm64/asmhelpers.asm	Removes stale imports and deletes an unused COM return helper.
src/coreclr/vm/arm/cgencpu.h	Removes unused ARM stack-probing APIs and related emitter helpers.
src/coreclr/vm/arm/asmhelpers.S	Removes permanently-disabled redirection and stack-probe stub code.
src/coreclr/vm/amd64/patchedcode.S	Replaces placeholder barrier body with fixed-size buffer fill.
src/coreclr/vm/amd64/patchedcode.asm	Same fixed-size buffer change for MASM build.
src/coreclr/vm/amd64/getstate.asm	Deletes unused AMD64 helpers (GetRBP, VM FPFillR8, get_cycle_count).
src/coreclr/vm/amd64/asmconstants.h	Introduces JIT_WRITE_BARRIER_BUFFER_SIZE constant (0x100).

Review details

• Files reviewed: 17/17 changed files
• Comments generated: 0
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

Cross-architecture assembly removals require human validation, and one stale cross-file comment remains.

Review details

Suppressed comments (1)

src/coreclr/vm/amd64/getstate.asm:25

• Removing FPFillR8 here leaves src/coreclr/debug/di/amd64/FloatConversion.asm:4-8 claiming that the duplicate implementation still exists in this file and must remain in sync. Please remove or update those comments as part of this cleanup so they do not point maintainers at nonexistent code.

        end

• Files reviewed: 14/14 changed files
• Comments generated: 0 new
• Review effort level: Balanced

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