[jdk17u-dev] RFR: 8313322: RISC-V: implement MD5 intrinsic

[Ilya Gavrilin]

Please review this backport of md5 intrinsics to jdk17u-dev.

Backport is not clean because:

- There was no no iterator for `class AbstractRegSet`, as an example `RegSetIterator` from aarch64 used: https://github.com/openjdk/jdk17u-dev/blob/0ddf1ebe3df119be2029b12feee93d0128e7cae0/src/hotspot/cpu/aarch64/register_aarch64.hpp#L379-L421
- Method `uint AbstractRegSet::size()` was missing
- Also for `size()` method we need `population_count()` function, so included `population_count.hpp` in `register_riscv.hpp`
- MD5 generation requires huge `codeBuffer`, with old codeBuffer sizes got error during intrinsic generation, so extended them:

  code_size1: 19000 -> 20000
  code_size2: 28000  -> 30000

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Commit messages:
 - Backport md5 sum to jdk17

Changes: https://git.openjdk.org/jdk17u-dev/pull/1739/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1739&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313322
  Stats: 463 lines in 6 files changed: 456 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1739.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1739/head:pull/1739

PR: https://git.openjdk.org/jdk17u-dev/pull/1739