ggml-amx : fix ggml_amx_init() on generic Linux (#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 <angt@huggingface.co>

ggml/src/ggml-cpu/amx/amx.cpp

@@ -7,7 +7,7 @@
 #include "ggml-cpu.h"
 #include "traits.h"
 
-#if defined(__gnu_linux__)
+#if defined(__linux__)
 #include <sys/syscall.h>
 #include <unistd.h>
 #endif
@@ -186,7 +186,7 @@ static size_t ggml_backend_amx_buffer_type_get_alloc_size(ggml_backend_buffer_ty
 #define XFEATURE_XTILEDATA      18
 
 static bool ggml_amx_init() {
-#if defined(__gnu_linux__)
+#if defined(__linux__)
     if (syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA)) {
         fprintf(stderr, "AMX is not ready to be used!\n");
         return false;
@@ -194,6 +194,8 @@ static bool ggml_amx_init() {
     return true;
 #elif defined(_WIN32)
     return true;
+#else
+    return false;
 #endif
 }