RFR: 8318827: RISC-V: Improve readability of fclass result testing

[Feilong Jiang]

Hi, please consider.

Currently, we test results of `fclass` instruction with hard-coded bits which has bad readability. This patch adds an enumeration of the flcass mask bits for ease of use.

Testing:

- [ ] tier1 with release build

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Commit messages:
 - Add FCLASS_MASK enum for better readability

Changes: https://git.openjdk.org/jdk/pull/16362/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16362&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318827
  Stats: 32 lines in 4 files changed: 19 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/16362.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16362/head:pull/16362

PR: https://git.openjdk.org/jdk/pull/16362