[foreign-abi] RFR: 8249803: Add LibraryLookup.Symbol [v2]

[Maurizio Cimadamore]

> This patch adds the LibraryLookup.Symbol, which implements Addressable, and that is returned on a successful symbol
> lookup. This patch also tweaks VaList to also implement the Addressable interface (VaList already had an `address()`
> method).  The `ForeignLinker::downcallHandle` method has been tweaked to accept an Addressable as a function entry
> point. This allows downcall handle to target not only native symbols, but also upcall stubs (since upcall stubs are
> segments, which are addressable) as well as native memory addresses.  Some tests needed minor tweaks.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/256/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/256/files/c9588de1..dc5751d5

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/256/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/256/webrev.00-01

  Stats: 11 lines in 2 files changed: 7 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/256.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/256/head:pull/256

PR: https://git.openjdk.java.net/panama-foreign/pull/256