Bring `IL_{Throw,Rethrow,ThrowExact}` to `PUSH_COOP_PINVOKE_FRAME` plan

[am11]

Follow up tracking issue from discussion in #116085. It will ultimately get rid of 1 of 6 remaining calls to HP-libunwind.

[am11]

cc @filipnavara, @davidwrighton

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[am11]

@filipnavara (based on the comments) would it be something like this:

               ---------------------
              |  Unified Fast IL_Throw |
               ---------------------
                         ▲
             ┌───────────┴────────────┐
             │                        │
  Uses for normal         Uses in transitions from
   managed throws            unmanaged (QCall, etc)
             │                        │
     [#1] PUSH_COOP_           [#2] Delay throw until
       PINVOKE_FRAME              managed reentry
                                    │
                         [#3] Thread-local signaling
                            (à la Objective-C)

or both paths with PUSH_COOP_PINVOKE_FRAME?

[filipnavara]

I'll need to look into it again. I realized I never got the floating point context quite right. There may not be anything dependent on that in the managed unwinding but I am less sure about the odd cases and C++/CLI.

The Obj-C path will eventually go through the same path in the end. I'd prefer if it's implemented through managed throw that then flows to the unified path on the left...