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.

depends/builders/darwin.mk

@@ -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