[foreign-memaccess+abi] RFR: 8294621: Copying from a StringCharBuffer-backed MemorySegment results in crash [v4]

[Per Minborg]

> This PR prevents `StringCharBuffer` arguments in `MemorySegment::ofBuffer`.

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

  Use more general determination if backed by array

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/741/files
  - new: https://git.openjdk.org/panama-foreign/pull/741/files/f5f1ec47..558db041

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=741&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=741&range=02-03

  Stats: 7 lines in 2 files changed: 1 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/741.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/741/head:pull/741

PR: https://git.openjdk.org/panama-foreign/pull/741