Fix passing Vector3s from the interpreter to native code

[kg]

Currently if you pass a VT argument with a size that is not a multiple of 8, the interp->native transition will misalign its stack offset and start loading garbage into registers after it processes the VT. The native->interp transition is handled correctly because the Store_Ref_XXX helpers already align offsets. The Load_Ref_Reg helpers don't do alignment, unlike Load_Stack_Ref which does.

This PR tweaks the callstub generator so that when generating a list of load routines, it aligns up the size of any VTs to maintain the alignment of the stack offsets. This is unnecessary for stack refs but harmless there. Store routines are left alone.

This PR also adds a new DOTNET_InterpHaltOnCall environment var that lets you make the interpreter assert when compiling calls to a specific method. It was helpful to have this when investigating this issue.

This fixes an issue in JIT\SIMD\Vector3Interop_ro\Vector3Interop_ro (but it still doesn't pass, due to a GC hole much later in the test)

[Copilot]

Pull Request Overview

This PR fixes a stack alignment issue in the CoreCLR interpreter when passing Vector3 arguments from interpreter to native code. The issue occurs when value types (VTs) with sizes not divisible by 8 bytes cause misaligned stack offsets, leading to garbage data being loaded into registers.

• Aligns VT argument sizes to 8-byte boundaries for interpreter-to-native transitions
• Adds debugging support via DOTNET_InterpHaltOnCall environment variable
• Maintains correct behavior for native-to-interpreter transitions which already handle alignment

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/callstubgenerator.cpp	Implements stack alignment fix for VT arguments in interpreter-to-native calls
src/coreclr/interpreter/interpconfigvalues.h	Adds configuration for InterpHaltOnCall debugging feature
src/coreclr/interpreter/compiler.cpp	Implements assertion trigger for InterpHaltOnCall debugging

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[janvorli]

LGTM, thank you!