Add riscv fallback detection

[carlosqwqqwq]

Why

simdzone currently ships x86_64-specific SIMD kernels for Westmere and Haswell, plus a generic fallback parser. That fallback is the right conservative baseline for riscv64 until a dedicated RISC-V SIMD backend exists. However, src/isadetection.h still checks raw x64 target macros directly, so forced-target validation on an x86_64 host can incorrectly enter the cpuid-based x64 detection path instead of the generic fallback path expected for riscv64.

What changed

• Add an explicit __riscv branch to src/isadetection.h that returns DEFAULT, so RISC-V resolves to the existing fallback parser instead of inheriting x64 ISA detection.
• Update CMakeLists.txt to emit a clear configure-time status message when the target processor matches riscv*, documenting that only the fallback kernel is built.
• Update README.md to state that RISC-V and other non-x86_64 targets currently use the fallback kernel until a dedicated SIMD backend is added.

Verification

• Ran the native CMake/Ninja build successfully:
  • cmake -S . -B build-native -G Ninja -DBUILD_TESTING=OFF -DBUILD_DOCUMENTATION=OFF
  • cmake --build build-native --parallel 4
• Ran a real riscv64 cross build and install with dockcross/linux-riscv64:
  • cmake -S /repo -B /tmp/simdzone-build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_SYSTEM_NAME=Linux -DCMAKE_SYSTEM_PROCESSOR=riscv64 -DCMAKE_C_COMPILER="$CC" -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY -DBUILD_TESTING=OFF
  • cmake --build /tmp/simdzone-build --parallel 4
  • cmake --install /tmp/simdzone-build --prefix /tmp/simdzone-install
• Linked a minimal zone_parse_string consumer against the installed libzone.a:
  • "$CC" -std=c99 -O2 -static -I/tmp/simdzone-install/include simdzone_smoke.c /tmp/simdzone-install/lib/libzone.a -o /tmp/simdzone_smoke.riscv64
• Verified the produced binary is a real RISC-V ELF:
  • file /tmp/simdzone_smoke.riscv64
  • readelf -h /tmp/simdzone_smoke.riscv64
• Ran the binary under qemu-riscv64 successfully:
  • output: simdzone-ok count=1 ttl=3600 addr=192.0.2.1

Notes

This is a conservative portability patch. It does not add RVV or any dedicated RISC-V SIMD implementation. The goal is to make riscv64 route cleanly to simdzone's existing fallback kernel and to keep x86_64-specific ISA detection from leaking into RISC-V validation.

[wcawijngaards]

The code looks good. Thank you for the additional platform checks.