RFR: 8310656: RISC-V: __builtin___clear_cache can fail silently.

[Robbin Ehn]

Hi, please consider.

We recently had a bug where user were missing permissions to use this syscall.
Which caused crashing on, according to hs_err on things like "addi x11, x24, 0" with SIGILL.
If it fails it is even possible to execute valid but 'old' instruction which may not lead to a crash, instead the program misbehaves.

To avoid this mess I suggest that we first test the syscall during vm init and we use it directly.
This way we can make sure it never fails.

Tested failing syscall with qemu, tested t1 in qemu, t1 on jh7110 in-progress.

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Commit messages:
 - 8310656: RISC-V: __builtin___clear_cache can fail silently.

Changes: https://git.openjdk.org/jdk/pull/14670/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14670&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310656
  Stats: 129 lines in 3 files changed: 126 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14670.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14670/head:pull/14670

PR: https://git.openjdk.org/jdk/pull/14670