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

[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).

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Commit messages:
 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Fix QSort benchmark
 - Add scope check
 - Merge branch 'foreign-memaccess+abi' into native_symbol
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/589/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=589&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274602
  Stats: 237 lines in 31 files changed: 102 ins; 72 del; 63 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