[jdk19] RFR: 8289302: Restore CriticalJNINatives

[Robbin Ehn]

On Wed, 29 Jun 2022 12:10:35 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change restores the code in the JVM that implemented the internal JavaCritical or CriticalJNINatives functionality.  Customers are using this feature and there's no replacement yet for it.
> This change is a backout and merge of jDK-8258192: Obsolete the CriticalJNINatives flag.
> This change also adds the deprecation message that was missed, which I checked is okay with the customer.
> 
> Tested with tiers1-4.

Anyhow why not just add jrt leaf style calls (thread stays in Java, no transition) to our FFI story (with "hic sunt dracones" disclaimer) if this is something we need to support?
It should be pretty straight forward, at least for @JornVernee :)

What is the panama take on this?

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PR: https://git.openjdk.org/jdk19/pull/90