RFR: 8299817: [s390] AES-CTR mode intrinsic fails with multiple short update() calls

[Lutz Schmidt]

This PR addresses an issue in the AES-CTR mode intrinsic on s390. When a message is ciphered in multiple, small (< 16 bytes) segments, the result is incorrect.

This is not just a band-aid fix. The issue was taken as a chance to restructure the code. though still complicated, It is now easier to read and (hopefully) understand.

Except for the new jetreg test, the changes are purely s390. There are no side effects on other platforms. Issue-specific tests pass. Other tests are in progress. I will update this PR once they are complete. 

**Reviews and comments are very much appreciated.**

@backwaterred could you please run some "official" s390 tests? Thanks.

-------------

Commit messages:
 - 8299817: [s390] AES-CTR mode intrinsic fails with multiple short update() calls

Changes: https://git.openjdk.org/jdk/pull/11967/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299817
  Stats: 714 lines in 5 files changed: 513 ins; 61 del; 140 mod
  Patch: https://git.openjdk.org/jdk/pull/11967.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11967/head:pull/11967

PR: https://git.openjdk.org/jdk/pull/11967