ggml : add test for SVE and disable when it fails

[slaren]

I verified that this disables SVE when building on Snapdragon 8 Gen 3, which is otherwise incorrectly enabled with -mcpu=native (clang 19.1.6)

I don't have a machine with SVE to test if it is correctly enabled when supported, any help testing this would be appreciated.

[ggerganov]

Tentatively approving, although I don't have any SVE hardware to test this. If there is some cloud hosting with SVE-enabled machines, I can give this a try.

[angt]

SVE works and is detected on graviton4:

-- ARM detected
-- Performing Test GGML_COMPILER_SUPPORTS_FP16_FORMAT_I3E
-- Performing Test GGML_COMPILER_SUPPORTS_FP16_FORMAT_I3E - Failed
-- Performing Test GGML_MACHINE_SUPPORTS_dotprod
-- Performing Test GGML_MACHINE_SUPPORTS_dotprod - Success
-- Performing Test GGML_MACHINE_SUPPORTS_i8mm
-- Performing Test GGML_MACHINE_SUPPORTS_i8mm - Success
-- Performing Test GGML_MACHINE_SUPPORTS_sve
-- Performing Test GGML_MACHINE_SUPPORTS_sve - Success
-- ARM feature DOTPROD enabled
-- ARM feature SVE enabled
-- ARM feature MATMUL_INT8 enabled
-- ARM feature FMA enabled
-- ARM feature FP16_VECTOR_ARITHMETIC enabled
-- Adding CPU backend variant ggml-cpu: -mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3+dotprod+i8mm+sve