[riscv-port] RFR: 8278994: riscv: RVC support [v18]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Add an assertion for patchable jals' alignment
 - Polish comments: remove the 'RVC:' prefix
 - Polish `nmethod_entry_barrier` and RISC-V CAS related comments
 - Remove `nmethod_entry_barrier`-related things
 - Split c_ldsp and c_fldsp
 - Remove useless and polish comments
 - Move RVC code to the proper location after rebasing (#42)
 - Rename misc functions and change the positions of some comments
 - Remove remaining macros as discussions
 - Remain an 'minimum_alignment' unchanged
 - ... and 10 more: https://git.openjdk.java.net/riscv-port/compare/83e32f9f...37899306

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

Changes:
  - all: https://git.openjdk.java.net/riscv-port/pull/34/files
  - new: https://git.openjdk.java.net/riscv-port/pull/34/files/3496b133..37899306

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=17
 - incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=34&range=16-17

  Stats: 1413 lines in 103 files changed: 898 ins; 276 del; 239 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