RFR: 8347343: RISC-V: Unchecked zicntr csr reads

[Robbin Ehn]

Hi, please consider.

The rdinstret/rdcycle/rdtime are really useful and would be great to add in e.g. JFR.
But as of now they are not used and zicntr is not yet in hwprobe therefore this patch just make sure they can't be used.

If we need them before hwprobe we can add a flag for manually enabling them.

Sanity tested.

Thanks!

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Commit messages:
 - Check for extension

Changes: https://git.openjdk.org/jdk/pull/23003/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23003&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347343
  Stats: 34 lines in 3 files changed: 7 ins; 18 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/23003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23003/head:pull/23003

PR: https://git.openjdk.org/jdk/pull/23003