Introduce backend GUIDs (ggml/743)

* Introduce backend GUIDs

Initial proposed implementation of backend GUIDs
(Discussed in https://github.com/ggerganov/ggml/pull/741)

Hardcoded CPU backend GUID (for now)
Change ggml_backend_is_cpu logic to use GUID

* Remove redundant functions

Remove redundant functions `ggml_backend_i::get_name` and `ggml_backend_guid` which are not desired for future expansion

* Add spaces to match style

Co-authored-by: slaren <slarengh@gmail.com>

* Fix brace style to match

Co-authored-by: slaren <slarengh@gmail.com>

* Add void to () in function signature

Co-authored-by: slaren <slarengh@gmail.com>

* Add back ggml_backend_guid and make CPU_GUID a local static in ggml_backend_cpu_guid

* add guids to all backends

ggml-ci

---------

Co-authored-by: slaren <slarengh@gmail.com>

ggml.c

@@ -355,6 +355,10 @@ void ggml_fp32_to_fp16_row(const float * x, ggml_fp16_t * y, int n) {
     }
 }
 
+bool ggml_guid_matches(ggml_guid_t guid_a, ggml_guid_t guid_b) {
+    return memcmp(guid_a, guid_b, sizeof(ggml_guid)) == 0;
+}
+
 //
 // timing
 //