RFR: 8304888: Add dedicated VMProps for linker and fallback linker

[Jorn Vernee]

This patch adds a dedicated jtreg property to check which CABI is being used by the JDK, which can be used both to check whether the foreign linker is supported, and whether the fallback linker is being used. (and, possibly it can also be use to check for a particular ABI in case we want to add ABI specific tests).

Checking whether the foreign linker is supported currently requires using an unwieldy expression that checks if we are running on a platform that has a foreign linker port. Checking for the fallback linker currently uses `vm.flavor != "zero"` which is not always correct, since the fallback linker can also be used on other platforms which are not `zero`.

To initialize the property, VMProps directly reads `jdk.internal.foreig.CABI::current()`. Since this class is in an internal package, `--add-exports` flags are added as javac flags and VM flags for the extra prop definitions class.

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Depends on: https://git.openjdk.org/jdk/pull/13079

Commit messages:
 - use only jdk.CABI property
 - use extra props to check for linker
 - Add fallbackLinker vmprop

Changes: https://git.openjdk.org/jdk/pull/13429/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304888
  Stats: 99 lines in 50 files changed: 17 ins; 3 del; 79 mod
  Patch: https://git.openjdk.org/jdk/pull/13429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13429/head:pull/13429

PR: https://git.openjdk.org/jdk/pull/13429