RFR: 8346107: Generators: testing utility for random value generation

[Emanuel Peter]

For verification testing, it is often critical to generate "interesting" values, to provoke overflows, NaN, etc. And to generate these values in the correct distribution to trigger certain optimizations.

I would like to start a collection of such generators, that can then be used in testing.

The goal is to grow this collection in the future, and add new types. For example `byte`, `char`, `short`, or even `Float16`.

This will be helpful for the Template framework [JDK-8344942](https://bugs.openjdk.org/browse/JDK-8344942), but also other tests.

Related PR, for value verification: https://github.com/openjdk/jdk/pull/22715

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Commit messages:
 - double generators
 - more float generators
 - wip float generators
 - JDK-8346107

Changes: https://git.openjdk.org/jdk/pull/22716/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22716&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346107
  Stats: 1612 lines in 21 files changed: 1612 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22716.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22716/head:pull/22716

PR: https://git.openjdk.org/jdk/pull/22716