[riscv-port] RFR: 8279565: riscv: RVB: Add byte reverse instruction

[Feilong Jiang]

This PR has the following changes:
1. Implement byte-reverse instruction `rev8` of RISC-V BitManipulation Extension.
2. Code improvement of existing byte reverse methods. 
3. Merge assembler_riscv_b.hpp and assembler_riscv_v.hpp into assembler_riscv.hpp

New C2 instructions under riscv_b.ad are covered by the following JTREG tests:
- test/hotspot/jtreg/compiler/codegen/Test6431242.java
- test/hotspot/jtreg/compiler/c2/TestCharShortByteSwap.java

Hotspot and jdk tier1 tests on QEMU (with and without UseRVB) are passed without new failures.

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Commit messages:
 - 8279565: riscv: RVB: Add byte reverse instruction

Changes: https://git.openjdk.java.net/riscv-port/pull/42/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=42&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279565
  Stats: 1845 lines in 10 files changed: 877 ins; 865 del; 103 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/42.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/42/head:pull/42

PR: https://git.openjdk.java.net/riscv-port/pull/42