[foreign-preview] RFR: 8275648: Linker naming bikeshed

[Jorn Vernee]

Hi,

This PR renames several classes, functions, and files in the linker implementation. So far this has been avoided while the implementation was still being worked out. But now that we are moving to preview state, it is time to revisit these names.

Roughly speaking, the following name changes are applied:

  - ProgrammableInvoker           -> DowncallLinker
  - ProgrammableUpcallHandler -> UpcallLinker
  - 'native invoker'                      -> 'downcall stub'
  - 'optimzed upcall stub'            -> 'upcall stub'
  - OptimizedEntryBlob               -> UpcallStub
  - optimized_entry_frame           -> upcall_stub_frame

Then, some source files in hotspot are also renamed as follows:

  - universalNativeInvoker* -> downcallLinker*
  - universalUpcallHandler* -> upcallLinker*
  - foreign_globals*             -> foreignGlobals* (to match existing convention)
  
I've also fixed up some outdated comments.

Thanks,
Jorn

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Commit messages:
 - Rename everything

Changes: https://git.openjdk.java.net/panama-foreign/pull/654/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=654&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275648
  Stats: 1511 lines in 75 files changed: 642 ins; 646 del; 223 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/654.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/654/head:pull/654

PR: https://git.openjdk.java.net/panama-foreign/pull/654