RISC-V CMake test defaults clobber e2e --iree-llvmcpu-target-cpu-features

[TMahlatini]

Related: #24769, #24802

Problem

On RISC-V Linux CMake builds, iree_compile_flags_for_platform always appends RISCV64_TEST_DEFAULT_LLVM_FLAGS, including --iree-llvmcpu-target-cpu-features=+m,+a,+f,+d,+c,+zvl512b,+v. iree_bytecode_module then puts those platform flags after the test variant's own flags. LLVM last-wins, so any TARGET_CPU_FEATURES_VARIANTS feature set is discarded.

The first casualty is riscv_64:ime:+v,+zvl256b,+xsmtvdot. The e2e target is named _ime but the vmfb is generic RVV at VLEN=512 with no +xsmtvdot.

Repro

Current CMake, build-riscv. Ninja line for e2e_matmul_cpu_dt_uk_i8_i32_llvm-cpu_local-task_ime_matmul.vmfb:

--iree-llvmcpu-target-cpu-features=+v,+zvl256b,+xsmtvdot
--iree-llvmcpu-target-triple=riscv64
--iree-llvmcpu-target-abi=lp64d
--iree-llvmcpu-target-cpu-features=+m,+a,+f,+d,+c,+zvl512b,+v

Second flag wins. Same order with host iree-compile on a 16×32 i8 matmul (--iree-opt-data-tiling --iree-llvmcpu-enable-ukernels=all) dumps:

cpu_features = "+m,+a,+f,+d,+c,+zvl512b,+v"
native_vector_size = 128

Tile is generic RVV linalg.mmt4d 7×64, not IME 12×16×8.

With a complete ISA (+m,+a,+f,+d,+c,+v,+zvl256b,+xsmtvdot) the dump is:

cpu_features = "+m,+a,+f,+d,+c,+v,+zvl256b,+xsmtvdot"
native_vector_size = 64

Tile is IME 12×16×8.

Suggested fix

main...TMahlatini:iree:users/tmahlatini/riscv-cpu-features-declobber