[REFACTOR][FFI] Phase out legacy C API (apache#18010)

This PR phases out the legacy c api in favor of the new FFI C API.
Also removes the redirection sccafolding for registry.h

- include <tvm/runtime/registry.h> => include <tvm/ffi/function.h>
- include <tvm/runtime/c_runtime_api.h> => include <tvm/runtime/base.h>
- TVM_REGISTER_GLOBAL => TVM_FFI_REGISTER_GLOBAL

The cleanup will greatly simplify the overall FFI surface of the project
and allows us to move towards an unified clean API based on tvm ffi.

Author: tqchen

include/tvm/ffi/c_api.h

@@ -27,6 +27,13 @@
 #include <dlpack/dlpack.h>
 #include <stdint.h>
 
+// Macros to do weak linking
+#ifdef _MSC_VER
+#define TVM_FFI_WEAK __declspec(selectany)
+#else
+#define TVM_FFI_WEAK __attribute__((weak))
+#endif
+
 #if !defined(TVM_FFI_DLL) && defined(__EMSCRIPTEN__)
 #include <emscripten/emscripten.h>
 #define TVM_FFI_DLL EMSCRIPTEN_KEEPALIVE

include/tvm/ffi/function.h

@@ -787,7 +787,7 @@ class Function::Registry {
    * .set_body_typed(multiply); // will have type int(int, int)
    *
    * // will have type int(int, int)
-   * TVM_REGISTER_GLOBAL("sub")
+   * TVM_FFI_REGISTER_GLOBAL("sub")
    * .set_body_typed([](int a, int b) -> int { return a - b; });
    *
    * \endcode