[foreign-memaccess+abi] RFR: 8294970: Add linker option for saving thread-locals that the VM can overwrite [v3]

[Jorn Vernee]

> This patch adds a new linker option, called `preserveValue`, that can be used to save thread-local values such as `errno` and `GetLastError` that the VM might overwrite. It takes a list of names of the values to save. Values are saved into a MemorySegment passed by the user, that is allocated according to the layout associated with the returned `PreserveValue` instance.
> 
> The API for this is somewhat convoluted, as we have to account for:
>  1. a different JVM implementation might not overwrite the same thread-locals, or non at all. Such JVM implementations should be free to support a different set of names to save, or none at all.
>  2. it should be possible to save multiple values during a single call.
> 
> But, this API is also flexible enough for an implementation to choose any layout it wants for the MemorySegment into which the values are saved. In the current implementation it's just a set of ints, but this could be changed as well.
> 
> ---------
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Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  plain text mentions of preserve

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/742/files
  - new: https://git.openjdk.org/panama-foreign/pull/742/files/adcfbd6a..7789ae86

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=742&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=742&range=01-02

  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/742.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/742/head:pull/742

PR: https://git.openjdk.org/panama-foreign/pull/742