[RNG][ARMPL] Support using OpenRNG in place of ArmPL

[rafbiels]

Description

Introduce the possibility to use the open-source OpenRNG library instead of the one bundled in ArmPL to build the oneMath ArmPL RNG backend. It can be used in two ways:

1. If ArmPL is not found but ArmPL RNG backend is enabled, fall back automatically to search for and use OpenRNG.
2. User can request it explicitly with -DENABLE_ARMPL_OPENRNG=True

The benefit of the open-source version is that it also works on x86_64 CPUs, so the backend becomes more portable. It also enables users constrained to open-source projects to use the backend.

Introduce new CMake find module for OpenRNG using the standard wrappers. This means the usual system library directories will be searched and user can also specify -DOpenRNG_ROOT=<path> as with other CMake modules.

The only functional difference between OpenRNG and ArmPL is that the former does not (currently) provide any API for version checking, so the version check is skipped. This means the oneMath backend will not throw version-dependent unimplemented exceptions in case of OpenRNG even if the version used doesn't support the feature. This affects the uniform integer distributions which give faulty results in negative integer ranges in previous releases of OpenRNG/ArmPL.

Minor extra changes:

• do not run FindARMPL.cmake version check if ArmPL is not found
• enable RNG examples for the ArmPL backend with opencl:cpu device

Checklist

• Do all unit tests pass locally? Attach a log.
  Tested the below configurations where aarch64 is a ubuntu:24.04/arm64 docker image emulated on x86_64 using binfmt:
  • reference - head of develop branch, aarch64, ArmPL, DPC++, POCL CPU device:
    • ut-aarch64-dpcpp-armpl-pocl.ref.gz
    • total: 162, skipped: 28, run: 134, failed: 78, passed: 56
    • I'm using POCL 5.0.2 which doesn't support some DPC++ runtime calls to OpenCL Intel extensions, hence the failures
  • same config as reference but with my changes - aarch64, ArmPL, DPC++, POCL CPU device:
    • ut-aarch64-dpcpp-armpl-pocl.log.gz
    • total: 164, skipped: 28, run: 136, failed: 79, passed: 57
    • the 2 new tests are the RNG examples which are now enabled for opencl:cpu device
  • newly supported config - aarch64, OpenRNG, DPC++, POCL CPU device:
    • ut-aarch64-dpcpp-openrng-pocl.log.gz
    • total: 164, skipped: 24, run: 140, failed: 83, passed: 57
    • the extra 4 failures are from the uniform integer distribution as explained above
  • newly supported config - x86_64, OpenRNG, DPC++, Intel OpenCL CPU device:
    • ut-openrng-x86_64-intelocl.log.gz
    • total: 164, skipped: 24, run: 140, failed: 4, passed: 136
    • the 4 failures are from the uniform integer distribution as explained above
• Have you provided motivation for adding a new feature? Increased portability - see description.
• Have you added relevant tests? No new scope for testing - existing tests cover the new backend option.