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

[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`.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/831/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=831&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307461
  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