RFR: 8284196: RISC-V: Detect supported ISA extensions over cpuinfo [v3]

[Feilong Jiang]

> Currently, `elf_hwcap` for RISC-V only sets single-letter extension bit (e.g. IMAFD).
> As many standard multi-letter ISA extensions are ratified (e.g. Zba/Zbb/Zbc/Zbs),
> we should find a stable way to detect  these supported ISA extensions in JVM.
> [1] has proposed a way to parse supported extensions through /proc/cpuinfo
> or "riscv,isa" string of /sys/firmware/devicetree, we could detect supported extensions
> in the same way.
> 
> Here is an example of /proc/cpuinfo with multi-letter extensions from Ubuntu 20.04 in QEMU-SYSTEM:
> 
> 
> ubuntu at ubuntu:~$ uname -a
> Linux ubuntu 5.8.0-14-generic #16~20.04.3-Ubuntu SMP Mon Feb 1 16:33:19 UTC 2021 riscv64 riscv64 riscv64 GNU/Linux
> ubuntu at ubuntu:~$ cat /proc/cpuinfo 
> processor       : 0
> hart            : 2
> isa             : rv64imafdch_zicsr_zifencei_zihintpause_zba_zbb_zbc_zbs_sstc
> mmu             : sv48
> 
> 
> 1: http://lists.infradead.org/pipermail/linux-riscv/2021-November/010252.html
> 
> 
> Testing:
> - [x] `jdk/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseZihintpause -XX:+UseRVV -XX:+UseZicbop -XX:+UseZba -XX:+UseZbb -XX:+UseZbs -XX:+PrintFlagsFinal -version` with release build

Feilong Jiang has updated the pull request incrementally with one additional commit since the last revision:

  fix comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12343/files
  - new: https://git.openjdk.org/jdk/pull/12343/files/59c33e2f..8def2029

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12343&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12343&range=01-02

  Stats: 18 lines in 1 file changed: 2 ins; 5 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12343.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12343/head:pull/12343

PR: https://git.openjdk.org/jdk/pull/12343