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

[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 and auto-enable 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 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

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

Commit messages:
 - code adjustment
 - adjust comments
 - update copyright
 - add zihintpuase
 - fix calling of strdup
 - Detect isa extensions from cpuinfo isa string

Changes: https://git.openjdk.org/jdk/pull/12343/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12343&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284196
  Stats: 210 lines in 3 files changed: 130 ins; 65 del; 15 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