ggml : add backend registry / device interfaces to BLAS backend (llama/9752)

* ggml : add backend registry / device interfaces to BLAS backend

* fix mmap usage when using host buffers

ggml/include/ggml-backend.h

@@ -170,6 +170,7 @@ extern "C" {
 
     // Functions that may be obtained using ggml_backend_reg_get_proc_address
     typedef ggml_backend_buffer_type_t (*ggml_backend_split_buffer_type_t)(const float *);
+    typedef void (*ggml_backend_set_n_threads_t)(ggml_backend_t, int);
 
     //
     // Backend registry

ggml/include/ggml-blas.h

@@ -17,6 +17,8 @@ GGML_API bool ggml_backend_is_blas(ggml_backend_t backend);
 // for openblas and blis, this will also set the number of threads used for blas operations
 GGML_API void ggml_backend_blas_set_n_threads(ggml_backend_t backend_blas, int n_threads);
 
+GGML_API ggml_backend_reg_t ggml_backend_blas_reg(void);
+
 
 #ifdef  __cplusplus
 }