[foreign-memaccess+abi] RFR: 8302346: Lift upcall sharing mechanism to AbstractLinker [v3]

[Jorn Vernee]

> Lift the upcall sharing mechanism to AbstractLinker, where it can live next to the similar mechanism for down calls. This also allows the fallback linker to use this mechanism.
> 
> Instead of an upcall stub, AbstractLinker::arrangeDowncall now return an UpcallStubFactory, which is a callback accepting a target MethodHandle and Arena, which are then used to construct the actual upcall stub.
> 
> Perhaps the trickiest part of this patch is that we have to simulate the effects of `SharedUtils::adaptUpcallForIMR` on the target method type. I've added a new method in SharedUtils, called `computeUpcallIMRType` for that, which mimics the shape of the `adaptUpcallForIMR` method.
> 
> Since the shape of most of the arrangeUpcall methods was very similar, I factored them out into a helper method in SharedUtils as well.

Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'foreign-memaccess+abi' into Lazy_Upcalls
 - use fake MH for adaptation
 - fix zero
 - tweak visibility
 - polish3
 - polish2
 - polish
 - Make upcall creation lazy at the AbstractLinker level

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/791/files
  - new: https://git.openjdk.org/panama-foreign/pull/791/files/6bd745f8..aa5e075d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=791&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=791&range=01-02

  Stats: 47376 lines in 990 files changed: 17838 ins; 15404 del; 14134 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/791.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/791/head:pull/791

PR: https://git.openjdk.org/panama-foreign/pull/791