Fix initialization of t_MostRecentUMEntryThunkData in reverse pinvokes

[BrzVlad]

In reverse pinvokes, the interpreter receives the UMEntryThunkData in this TLS variable as a hidden argument. The way this is implemented is that reverse pinvokes always go through the prestub which checks whether the target is interpreter, in which case it writes this TLS variable. This means that first call was going through the prestub twice, first to initialize the thunk data and secondly to set this argument (TheUMEntryPrestubWorker was returning itself so it gets called again).

Following 60f1dc1, we obtain the entryPoint so we end up calling the interpreter precode directly. The problem is that we failed to set the TLS variable, which is what this PR is doing.

This also does minor cleanup by removing dead interpreter code from COM sources and renaming targetIsPrecode to canSkipPreStub, because the first name was misleading (it was false when entryPoint was pointing to interpreter precode).

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[Copilot]

Pull request overview

This PR fixes reverse P/Invoke behavior when targeting the interpreter by ensuring t_MostRecentUMEntryThunkData is populated even when the call path can directly jump to the interpreter precode (skipping the second prestub pass that previously performed the TLS write).

Changes:

• Update TheUMEntryPrestubWorker to set t_MostRecentUMEntryThunkData when the prestub cannot be skipped (interpreter path), including a rename from targetIsPrecode → canSkipPreStub for clarity.
• Remove interpreter-specific TLS plumbing from COM call prestub code, which is now dead/unnecessary.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/dllimportcallback.cpp	Ensures reverse P/Invoke interpreter path initializes the TLS thunk-data used as the hidden argument.
src/coreclr/vm/comcallablewrapper.cpp	Removes unused interpreter-specific TLS handling from COM prestub flow.