RISC-V (riscv64) support: workarounds needed to build O2 #15664

Building O2 on riscv64 works, but I need a few workarounds in our alidist recipe. Listing them here so we can maybe fix the causes in O2 and drop the hacks. I can send PRs for the easy ones.

Simple ones, just need a riscv64 branch added:

1. Framework/Foundation/3rdparty/x9/x9.c errors out on anything that isn't x86 or aarch64. RISC-V has a PAUSE hint we can emit as .4byte 0x0100000F, so adding a riscv64 branch fixes it. This one is vendored third-party, so it could also go upstream instead.

2. In Utilities/rANS/include/rANS/internal/common/defines.h the coder detection only knows x86 and aarch64, so riscv64 hits #error your hardware or compiler settings do not support librans. Adding riscv64 picks the existing scalar Compat coder and builds fine, no SIMD needed.

Ones that probably need a maintainer to decide:

3. In Utilities/rANS/CMakeLists.txt, with BUILD_TESTING=OFF the o2_add_test() call makes no target, but target_compile_options(${TEST_*}) still runs right after and config fails. This isn't riscv64 specific, it happens anywhere with testing off. Probably those lines should only run when the target exists.

4. ROOT on riscv64 has no asimage or opengl, so ROOT::Eve, ROOT::ASImage and ROOT::RGL are missing. That makes EventVisualisation, the MCH Geometry Test, and the ITS EVE macros fail to configure. They're all GUI things not needed for headless running, so right now I just skip those dirs. Is there a clean way to guard them on the ROOT GUI targets being available?
   Skipping those dirs then makes cmake/O2ReportNonTestedMacros.cmake fatal because their macros aren't tested anymore. I get around it by demoting the check. Would be nicer if it could ignore dirs that are turned off on purpose for a given arch or build.

Env: riscv64, Ubuntu 26.04, GCC 15, O2 daily-20260309-0000