message: build: Fix regression introduced in PR23603
  Assignments in builders/darwin.mk actually override previous assignments
  in hosts/default.mk. Therefore, the append operator must be used
  instead.
diff: @@ -22,7 +22,7 @@ darwin_INSTALL_NAME_TOOL:=$(shell xcrun -f install_name_tool)
  darwin_native_binutils=
  darwin_native_toolchain=
  -x86_64_darwin_CFLAGS = -arch x86_64
  -x86_64_darwin_CXXFLAGS = $(x86_64_darwin_CFLAGS)
  -aarch64_darwin_CFLAGS = -arch arm64
  -aarch64_darwin_CXXFLAGS = $(aarch64_darwin_CFLAGS)
  +x86_64_darwin_CFLAGS += -arch x86_64
  +x86_64_darwin_CXXFLAGS += -arch x86_64
  +aarch64_darwin_CFLAGS += -arch arm64
  +aarch64_darwin_CXXFLAGS += -arch arm64
  