[riscv-port] RFR: 8278994: riscv: RVC support [v19]

[Xiaolin Zheng]

> Hi team,
> 
> This patch includes the basic definition of the RVC instruction set and some cleanups. Tested a simple `test/hotspot/jtreg/compiler/` folder on qemu.
> 
> Using `<JAVA_HOME>/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseRVC -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:PrintAssemblyOptions=no-aliases,numeric -XX:+PrintStubCode -XX:-TieredCompilation` could show RVC instructions.
> 
> Thanks,
> Xiaolin

Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:

  Polish English comments as suggestions

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

Changes:
  - all: https://git.openjdk.java.net/riscv-port/pull/34/files
  - new: https://git.openjdk.java.net/riscv-port/pull/34/files/37899306..502ff3d8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=18
 - incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=17-18

  Stats: 14 lines in 4 files changed: 0 ins; 2 del; 12 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/34/head:pull/34

PR: https://git.openjdk.java.net/riscv-port/pull/34