[riscv-port] RFR: 8278994: riscv: RVC support [v2]

[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

Xiaolin Zheng has updated the pull request incrementally with three additional commits since the last revision:

 - Cover most RVC instructions by using CompressibleRegion to cover minimal functions in C2
 - Revise as proposed comments, including
   - Fix macros in assembler_riscv_c.hpp
   - Remove UncompressibleRegion
   - Modify comments
   - Change names: C-Ext to RVC
 - Enable RVC instructions (based on the basic patch)

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

Changes:
  - all: https://git.openjdk.java.net/riscv-port/pull/34/files
  - new: https://git.openjdk.java.net/riscv-port/pull/34/files/41ec7bd1..dbc763ed

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=00-01

  Stats: 622 lines in 13 files changed: 479 ins; 32 del; 111 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