fix(cmake): don't pass -fno-fat-lto-objects on Apple GCC (#6114)

GCC on macOS has no linker plugin, so `-fno-fat-lto-objects` makes every
LTO probe fail and LTO is silently disabled. Plain `-flto=auto` works.

Fixes #6060

Assisted-by: ClaudeCode:claude-opus-5
Claude-Session: https://claude.ai/code/session_013JABmnjt9oAh29p1pytAB3

tools/pybind11Common.cmake

diff --git a/tools/pybind11Common.cmake b/tools/pybind11Common.cmake
index 1311d3f..f525270 100644
--- a/tools/pybind11Common.cmake
+++ b/tools/pybind11Common.cmake
@@ -353,7 +353,11 @@
     if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE)
       # Clang Gold plugin does not support -Os; append -O3 to MinSizeRel builds to override it
       set(linker_append ";$<$<CONFIG:MinSizeRel>:-O3>")
-    elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND NOT MINGW)
+    elseif(
+      CMAKE_CXX_COMPILER_ID MATCHES "GNU"
+      AND NOT MINGW
+      AND NOT APPLE)
+      # GCC on macOS has no linker plugin, which -fno-fat-lto-objects requires
       set(cxx_append ";-fno-fat-lto-objects")
     endif()