ggml : fix ARM feature verification (llama/17519)

ggml : fix ARM feature verification (llama/17519)

On arm64 with `cmake` version 3.31.6, the final feature verification fails:

    -- ARM detected flags: -mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3+nossbs
    -- 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
    -- Performing Test GGML_MACHINE_SUPPORTS_sme
    -- Performing Test GGML_MACHINE_SUPPORTS_sme - Failed
    -- Performing Test GGML_MACHINE_SUPPORTS_nosme
    -- Performing Test GGML_MACHINE_SUPPORTS_nosme - Success
    -- Checking for ARM features using flags:
    --   -U__ARM_FEATURE_SME
    --   -mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3+nossbs+dotprod+i8mm+sve+nosme
    -- Performing Test HAVE_DOTPROD
    -- Performing Test HAVE_DOTPROD - Failed
    -- Performing Test HAVE_SVE
    -- Performing Test HAVE_SVE - Failed
    -- Performing Test HAVE_MATMUL_INT8
    -- Performing Test HAVE_MATMUL_INT8 - Failed
    -- Performing Test HAVE_FMA
    -- Performing Test HAVE_FMA - Success
    -- Performing Test HAVE_FP16_VECTOR_ARITHMETIC
    -- Performing Test HAVE_FP16_VECTOR_ARITHMETIC - Failed
    -- Performing Test HAVE_SME
    -- Performing Test HAVE_SME - Failed
    -- Adding CPU backend variant ggml-cpu: -U__ARM_FEATURE_SME;-mcpu=neoverse-v2+crc+sve2-aes+sve2-sha3+nossbs+dotprod+i8mm+sve+nosme

We need to explicitly replace `;` with spaces from the list to make
`CMAKE_REQUIRED_FLAGS` work correctly...

Signed-off-by: Adrien Gallouët <redacted>