[foreign-memaccess+abi] RFR: 8317514: Ensure MemorySegment is initialized before touching NativeMemorySegmentImpl [v2]

[Jorn Vernee]

> Please review this fix for a class initializer deadlock.
> 
> The issue is that `MemorySegment` references `NativeMemorySegmentImpl` in its initializer, through the `NULL` constant. `NativeMemorySegmentImpl` also depends on `MemorySegment` to be initialized, since it's a super interface with default methods. The VM does not drop the initialization monitor of `NativeMemorySegmentImpl` while trying to initialize super classes/interfaces (as mandated by the JVM spec).
> 
> When 2 threads try to intialize `MemorySegment` (e.g. through `ofAddress`) and `NativeMemorySegmentImpl` (e.g. through Arena::allocateFrom), at the same time, a deadlock can occur.
> 
> This PR fixes this issue by ensuring `MemorySegment` is initialized in all cases where we directly access `NativeMemorySegmentImpl` or `MappedMemorySegmentImpl`.
> 
> We might consider a fix that removes the cycle in the future as well.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Add test for FileChannel::map

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/902/files
  - new: https://git.openjdk.org/panama-foreign/pull/902/files/22c824d4..cccdebf4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=902&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=902&range=00-01

  Stats: 46 lines in 1 file changed: 31 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/902.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/902/head:pull/902

PR: https://git.openjdk.org/panama-foreign/pull/902