[foreign-memaccess+abi] RFR: Fix sonar issues and thread safety bug [v2]

[Per Minborg]

> This PR proposes some fix of Sonar warnings and a thread safety issue.

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

  Revert changes to FFIType

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/906/files
  - new: https://git.openjdk.org/panama-foreign/pull/906/files/1b19f91a..277a9dda

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=906&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=906&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/906.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/906/head:pull/906

PR: https://git.openjdk.org/panama-foreign/pull/906