[foreign-memaccess+abi] RFR: 8308293: A linker should expose the layouts it supports

[Maurizio Cimadamore]

This is another stab at https://github.com/openjdk/jdk/pull/14037

I believe, after some offline discussion, that we have found a more satisfying solution to the problem of JAVA_CHAR being exposed. Jorn suggested that linkers should also provide mappings for JNI types such as `jint`, `jshort` and such (which are aliases for our layout constants anyway). I think that's a great way to bring `JAVA_CHAR` back into the fold.

For now, I decided not to specify support for JNI canonical layouts (but I could do so, if that's preferred). I think the highest priority is to provide some stable mappings for C builtin types (+ `size_t`) as that's what 99% of developers will be struggling with.

API-wise, we just expose a map. In the preovious PR there were questions as to whether the map should be split into two methods. In general I see the following options:

1. Just expose a map (that's the primitive, other things can derived from it)
2. Expose a map, plus a method to get a canonical layout from a type name (that's the `Charset` approach, which has both `availableCharsets` *and* `forName`)
3. Expose a method to get canonical layout from name, plus a method that returns the set of supported canonical layout names

My (not so strong) preference would be for either (1) or (2).

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/839/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=839&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308293
  Stats: 114 lines in 2 files changed: 76 ins; 18 del; 20 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/839.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/839/head:pull/839

PR: https://git.openjdk.org/panama-foreign/pull/839