RFR: 8368732: RISC-V: Detect support for misaligned vector access via hwprobe [v4]

[Fei Yang]

> Hi, this is a followup change after [JDK-8368366](https://bugs.openjdk.org/browse/JDK-8368366).
> 
> According to the latest RISC-V linux hardware probing syscall [1], the performance of misaligned memory accesses has been divided into two cases: `RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF` and `RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF` for scalar and vector respectively. 
> 
> This aligns `AlignVector` with `RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF`. That is if the misaligned vector access is fast, we set `AlignVector` to false in the hope that it will save instructions handling address alignment thus improves performance.
> 
> This choose to keep the use of `RISCV_HWPROBE_KEY_CPUPERF_0` which is now deprecated and returns similar values
> to `RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF` for backward compatibility with old linux kernels.
> 
> Manually checked the result on platforms w/wo fast misaligned vector accesses by running: 
> `$java -XX:+PrintFlagsFinal -version | grep AlignVector`
> 
> [1] https://docs.kernel.org/arch/riscv/hwprobe.html

Fei Yang has updated the pull request incrementally with one additional commit since the last revision:

  Fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27512/files
  - new: https://git.openjdk.org/jdk/pull/27512/files/444bfc35..a80ee075

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27512&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27512&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27512.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27512/head:pull/27512

PR: https://git.openjdk.org/jdk/pull/27512