[foreign-abi] Integrated: Turn off down call intrinsics by default

[Jorn Vernee]

Hi,

Since there were some downstream failures in the jextract samples, this patch turns of the down call intrinsics by
default, at least until the problem can be investigated and fixed. It also changes the names of the used system
properties from NO_XXX -> USE_XXX to make them a little easier to understand.

Thanks,
Jorn

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Commit messages:
 - Turn off down call intrinsics by default

Changes: https://git.openjdk.java.net/panama-foreign/pull/229/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/229/webrev.00
  Stats: 43 lines in 4 files changed: 26 ins; 5 del; 12 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/229.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/229/head:pull/229

PR: https://git.openjdk.java.net/panama-foreign/pull/229