Add support for DR_CLEANCALL_MULTIPATH + DR_CLEANCALL_WRITES_APP_CONTEXT

Split from #4128 (closed).

Supporting multipath with writing requires extra drreg functionality for stateless respills, but given that we have not seen any code that requires it we have not put the effort into supporting it.

It is possible that the component inserting the multi-path clean call is not the one controlling the drreg usage and so would have a hard time inserting manual restores.

This issue covers this corner case.