[foreign-memaccess+abi] RFR: 8307461: Linker::nativeLinker should not be restricted [v2]

[Maurizio Cimadamore]

> Since `Linker` is a sealed interface, we can now move restricted-ness from the static linker factory, down to the methods which are really problematic, namely `Linker::downcallHandle` and `Linker::upcallStub`.
> 
> With this change, the list of restricted method looks as follows:
> 
> * Linker::downcallHandle
> * Linker::upcallStub
> * MemorySegment::reinterpret
> * SymbolLookup::libraryLookup
> 
> That is, crucially, restricted methods are methods that have a true risk of compromising the integrity of the Java platform. Making Linker::nativeLinker restricted casts too broad of a net, as obtaining the linker is not, per se, a dangerous or unsafe operation.

Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'foreign-memaccess+abi' into restricted_linker
 - Initial push

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

Changes: https://git.openjdk.org/panama-foreign/pull/831/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=831&range=01
  Stats: 123 lines in 11 files changed: 60 ins; 17 del; 46 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/831.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/831/head:pull/831

PR: https://git.openjdk.org/panama-foreign/pull/831