ggml-amx : fix ggml_amx_init() on generic Linux (llama/16049)

ggml-amx : fix ggml_amx_init() on generic Linux (llama/16049)

Generalize Linux check to `__linux__` to support non-glibc systems (like musl).
Also, return `false` on unknown/untested OS.

Without this commit, the code compiles (with warnings) but fails:

    register_backend: registered backend CPU (1 devices)
    register_device: registered device CPU (Intel(R) Xeon(R) Platinum 8488C)
    build: 6487 (51c4cac6) with x86_64-linux-musl-gcc (GCC) 15.1.0 for x86_64-linux-musl (debug)
    system info: n_threads = 8, n_threads_batch = 8, total_threads = 16
    ....
    print_info: n_ctx_orig_yarn  = 262144
    print_info: rope_finetuned   = unknown
    print_info: model type       = 4B
    Illegal instruction (core dumped)

Signed-off-by: Adrien Gallouët <redacted>