[foreign-memaccess+abi] RFR: 8292078: Thread-safety issues in AbstractLinker implementations [v4]

[Per Minborg]

> Change to lazy static initializing for singletons and fix a bug in `LinuxAArch64VaList` that threw an `IndexOutOfBoundsException`.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused imports

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/702/files
  - new: https://git.openjdk.org/panama-foreign/pull/702/files/37558149..1adad8fc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=702&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=702&range=02-03

  Stats: 3 lines in 3 files changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/702.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/702/head:pull/702

PR: https://git.openjdk.org/panama-foreign/pull/702