RFR: 8268129: LibraryLookup::ofDefault leaks symbols from loaded libraries

[Maurizio Cimadamore]

This patch overhauls the library loading mechanism used by the Foreign Linker API. We realized that, while handy, the *default* lookup abstraction (`LibraryLookup::ofDefault`) was behaving inconsistentlt across platforms.

This patch replaces `LibraryLookup` with a simpler `SymbolLookup` abstraction, a functional interface. Crucially, `SymbolLookup` does not concern with library loading, only symbol lookup. For this reason, two factories are added:

* `SymbolLookup::loaderLookup` - which obtains a lookup that can be used to lookup symbols in libraries loaded by current loader
* `CLinker::systemLookup` - a more stable replacement for the *default* lookup, which looks for symbols in libc.so (or its equivalent in other platforms). The contents of this lookup are unspecified.

Both factories are *restricted*, so they can only be called when `--enable-native-access` is set.

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Commit messages:
 - Improve javadoc
 - Initial push

Changes: https://git.openjdk.java.net/jdk/pull/4316/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4316&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268129
  Stats: 1278 lines in 44 files changed: 569 ins; 617 del; 92 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4316.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4316/head:pull/4316

PR: https://git.openjdk.java.net/jdk/pull/4316