[foreign-preview] RFR: 8276648: Downcall stubs are never freed

[Jorn Vernee]

Hi,

The current code adds native invokers (downcall stubs) to a CocurrentHashMap cache, and they stay there until the application exits.

This patch replaces that cache with a SoftReferenceCache (brought to the top level from AbstractLinker), which allows at least the values to be evicted when they become unreachable.

Those value, which are NativeEntryPoint instances, are also registered with a cleaner, whose cleanup action will free the stub from the code cache.

Thanks,
Jorn

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Commit messages:
 - Use SoftReferenceCache for NativeEntryPoint and free stubs with a cleaner

Changes: https://git.openjdk.java.net/panama-foreign/pull/651/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=651&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276648
  Stats: 135 lines in 5 files changed: 88 ins; 35 del; 12 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/651.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/651/head:pull/651

PR: https://git.openjdk.java.net/panama-foreign/pull/651