remove -Ofast for Clang to fix AppleClang deprecation warnings

AppleClang 17 is based on LLVM Clang 19, but its version number is numerically less than 19.0.
Since `-Ofast` in Clang is equivalent to `-O3 -ffast-math`, `-Ofast` is redundant. This commit disables `-Ofast` for all Clang-based compilers (including AppleClang) to resolve the version mismatch and avoid future deprecation issues.

Author: zhuzeitou

src/CMakeLists.txt

@@ -372,7 +372,7 @@ else()
     endif()
 
     if(CMAKE_BUILD_TYPE MATCHES "(Release|RELEASE|release)")
-        if(NOT CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.6) AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0)))
+        if(NOT CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.6) AND NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
             target_compile_options(ncnn PRIVATE -Ofast)
         endif()