[foreign-jextract] RFR: 8266814: Improve library loading with SymbolLookup abstraction [v2]

[Maurizio Cimadamore]

> This patch add jextract support for the newly added `SymbolLookup` abstraction.
> 
> Note that the symbol lookup used by jextract classes is a *chained* lookup. That is, first the loader lookup is searched, and, when that fails, the system lookup is also searched.

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

  Address review comments

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/532/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/532/files/6b8d8c56..c13a5e0a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=532&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=532&range=00-01

  Stats: 7 lines in 2 files changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/532.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/532/head:pull/532

PR: https://git.openjdk.java.net/panama-foreign/pull/532