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

[Per Minborg]

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

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Commit messages:
 - Fix thread issues and IOOBE issues

Changes: https://git.openjdk.org/panama-foreign/pull/702/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=702&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292078
  Stats: 65 lines in 5 files changed: 19 ins; 3 del; 43 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