Detect riscv64 targets and disable JIT

[carlosqwqqwq]

Why

melonDS used host compiler predefined macros for architecture selection, so a simulated or cross-target riscv64 configuration could still inherit the host x86_64 JIT path. This patch makes the riscv64 baseline path explicit and keeps RISC-V on the interpreter-only core build.

What changed

• Recognize CMAKE_SYSTEM_PROCESSOR=riscv64 explicitly in the top-level CMake architecture selection instead of relying on host compiler predefined macros.
• Route RISC-V 64-bit targets onto the interpreter-only core build path and keep ENABLE_JIT off by default.
• Preserve the existing x86_64 and ARM64 JIT paths without changing any JIT backend implementation.
• Keep this patch conservative: it does not add a RISC-V JIT backend or RISC-V-specific graphics optimizations.

Verification

• Ran native CMake configuration with Ninja on the local Windows host using BUILD_QT_SDL=OFF, ENABLE_OGLRENDERER=OFF, TEAKRA_BUILD_UNIT_TESTS=OFF, and TEAKRA_BUILD_TOOLS=OFF.
• Built the native tree successfully with cmake --build build-codex-native-riscvfix --parallel 4.
• Ran simulated riscv64 CMake configuration with CMAKE_SYSTEM_NAME=Linux, CMAKE_SYSTEM_PROCESSOR=riscv64, CMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY, BUILD_QT_SDL=OFF, ENABLE_OGLRENDERER=OFF, TEAKRA_BUILD_UNIT_TESTS=OFF, TEAKRA_BUILD_TOOLS=OFF, and forced __riscv=1 / __riscv_xlen=64 macros because no real RISC-V toolchain is available on the host.
• Built the simulated riscv64 tree successfully with cmake --build build-codex-riscv-riscvfix --parallel 4.
• Confirmed the simulated riscv64 configuration prints Detected RISC-V 64-bit target; building interpreter path with JIT disabled.
• Checked build-codex-riscv-riscvfix/build.ninja and compile_commands.json: they contain ARCHITECTURE_RISCV64=1, __riscv=1, and __riscv_xlen=64.
• Confirmed build-codex-riscv-riscvfix/build.ninja does not contain ARCHITECTURE_x86_64=1, JIT_ENABLED, ARMJIT_x64, ARMJIT_A64, x64Emitter, Arm64Emitter, GPU_OpenGL, or OGLRENDERER_ENABLED.

Notes

This is a conservative portability patch. It only fixes target architecture selection so that riscv64 no longer inherits the host x86_64 JIT path during configuration. Verification covers native core-only build stability plus simulated RISC-V target selection and backend exclusion checks, but not a real riscv64 cross-toolchain build, frontend dependency build, or hardware run.