RFR: 8345296: AArch64: VM crashes with SIGILL when prctl is disallowed [v4]

[Aleksey Shipilev]

> We have caught this in some prod environments, where `prctl` is forbidden by the sandboxing mechanism. This fails the JVM, because we have the following code to check for SVE vector length:
> 
> 
> int VM_Version::get_current_sve_vector_length() {
>   assert(VM_Version::supports_sve(), "should not call this");
>   return prctl(PR_SVE_GET_VL);
> }
> 
> 
> That code returns `-1` when `prctl` is disallowed, which JVM then blindly interprets as vector length, leading to `SIGILL`. I looked around other uses of `prctl` around Hotspot, and they all seem to handle the errors correctly.
> 
> Additional testing:
>  - [x] Linux AArch64 server fastdebug, with seccomp reproducer
>  - [ ] Linux AArch64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Simplify the condition: zero is handled by is_power_of_two

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22479/files
  - new: https://git.openjdk.org/jdk/pull/22479/files/5777379a..16bde2b1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22479&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22479&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22479/head:pull/22479

PR: https://git.openjdk.org/jdk/pull/22479