[foreign-memaccess+abi] RFR: 8274602: Generalize UpcallStub into NativeSymbol [v2]

[Maurizio Cimadamore]

> It might sometimes be useful to define a custom symbol lookup which returns symbols backed by a library that can be unloaded dynamically (e.g. using dlopen).
> We indeed have all the pieces to get there, except that symbnol lookup returns a memory address, which is not a scoped entity.
> 
> To address this, I propose that we add a new sealed interface/final class (`NativeSymbol`) which implements `Addressable` and is used in two places:
> * as a return value for a symbol lookup
> * as a proxy for an upcall stub
> 
> In other words, `NativeSymbol` is used to reference to some symbol in some library, where the library can have a given scope. Upcall stubs are just a special case where the upcall symbol is synthetized on the fly by the VM. With this in place, I think we can drop `CLinker.UpcallStub` and just use an "anonymous` NativeSymbol to represent upcall stubs.
> 
> With this, it should be possible to define a dlopen-based lookup such as the one below:
> https://github.com/sundararajana/panama-jextract-samples/blob/master/dlopen/Dlopen.java
> 
> In a way that is completely *safe* - that is, so that it is not possible for a library to be unloaded while a function is being executed (even when the scope  associated to the native symbol is shared).

Maurizio Cimadamore 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 ten additional commits since the last revision:

 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Use NativeSymbol in downcallHandle
 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Add missing copyright headers
 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Fix QSort benchmark
 - Add scope check
   Add test
 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Initial push

-------------

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/589/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/589/files/123fb4ab..3302f2fb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=589&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=589&range=00-01

  Stats: 7553 lines in 325 files changed: 5366 ins; 1163 del; 1024 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/589.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/589/head:pull/589

PR: https://git.openjdk.java.net/panama-foreign/pull/589