build: Fix x86_64 <-> arm64 cross-compiling for macOS

Author: hebasto

depends/builders/darwin.mk

@@ -21,3 +21,8 @@ darwin_NM:=$(shell xcrun -f nm)
 darwin_INSTALL_NAME_TOOL:=$(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)