[riscv-port] RFR: 8278994: riscv: RVC: basic instruction set

[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.

Thanks,
Xiaolin

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Commit messages:
 - RVC: basic instruction set

Changes: https://git.openjdk.java.net/riscv-port/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278994
  Stats: 627 lines in 8 files changed: 607 ins; 3 del; 17 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