[foreign-memaccess+abi] RFR: 8312981: Make the linker API required

[Jorn Vernee]

Currently the Linker::nativeLinker method is specified to throw an UnsupportedOperationException when the JDK does not include a linker implementation.

This patch drops this exception specification, and effectively requires all JDK implementations to implement the linker. This move is vital to allow the JDK to start using the linker API as well.

It also turns on all the tests by default. This will mean that implementations that don't conform (because they are missing a linker) will get a useful indication of that fact through test failures.

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Commit messages:
 - update linker spec
 - enable tests by default

Changes: https://git.openjdk.org/panama-foreign/pull/850/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312981
  Stats: 77 lines in 49 files changed: 0 ins; 77 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/850.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/850/head:pull/850

PR: https://git.openjdk.org/panama-foreign/pull/850