[clr-interp] Fix interop calls when PInvokePrecodeImport thunks are involved

[davidwrighton]

• Handle the case where the dispatch may get upgraded from a PInvokeImportThunk to a direct function pointer

[Copilot]

Pull Request Overview

This PR fixes interop calls in the CLR interpreter when PInvokePrecodeImport thunks are involved by handling cases where the dispatch method may be upgraded from a PInvokeImportThunk to a direct function pointer during multi-threaded execution.

Key changes:

• Adds logic to detect and handle PInvokeImportPrecode targets
• Updates target validation to accommodate both precode and direct function pointers
• Implements thread-safe target updates using volatile operations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/vm/interpexec.cpp	Adds GetTargetPInvokeMethodDesc function and updates InvokeManagedMethod to handle PInvokeImportPrecode race conditions
src/coreclr/vm/callstubgenerator.h	Makes SetTarget and GetTarget operations thread-safe using volatile store/load

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