mac: fix crash on old macos versions

cblas_sgemm is only supported on v13.3 and up, however bf16 is
only supported on v14+ so we were falling back to ggml-blas and
crashing on bf16 tensors.  Checking for the function being null
seems to be the simplest way to condittionally avoid registering the
backend.

llama/patches/0025-Disable-ggml-blas-on-macos-v13-and-older.patch

+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Daniel Hiltgen <daniel@ollama.com>
+Date: Sun, 3 Aug 2025 10:00:20 -0700
+Subject: [PATCH] Disable ggml-blas on macos v13 and older
+
+---
+ ggml/src/ggml-blas/ggml-blas.cpp | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/ggml/src/ggml-blas/ggml-blas.cpp b/ggml/src/ggml-blas/ggml-blas.cpp
+index ec158dfa..22926d75 100644
+--- a/ggml/src/ggml-blas/ggml-blas.cpp
++++ b/ggml/src/ggml-blas/ggml-blas.cpp
+@@ -505,6 +505,11 @@ static const struct ggml_backend_reg_i ggml_backend_blas_reg_i = {
+ };
+ 
+ ggml_backend_reg_t ggml_backend_blas_reg(void) {
++    // MacOS prior to v14 does not include cblas_sgemm - disable this backend if it isn't available
++    if (&cblas_sgemm == NULL) {
++        GGML_LOG_INFO("Disabling ggml-blas backend on old MacOS version\n");
++        return NULL;
++    }
+     static struct ggml_backend_reg ggml_backend_blas_reg = {
+         /* .api_version = */ GGML_BACKEND_API_VERSION,
+         /* .iface       = */ ggml_backend_blas_reg_i,

ml/backend/ggml/ggml/src/ggml-blas/ggml-blas.cpp

@@ -505,6 +505,11 @@ static const struct ggml_backend_reg_i ggml_backend_blas_reg_i = {
 };
 
 ggml_backend_reg_t ggml_backend_blas_reg(void) {
+    // MacOS prior to v14 does not include cblas_sgemm - disable this backend if it isn't available
+    if (&cblas_sgemm == NULL) {
+        GGML_LOG_INFO("Disabling ggml-blas backend on old MacOS version\n");
+        return NULL;
+    }
     static struct ggml_backend_reg ggml_backend_blas_reg = {
         /* .api_version = */ GGML_BACKEND_API_VERSION,
         /* .iface       = */ ggml_backend_blas_reg_i,