Fix the Debian package architecture for arm64

The system triplet uses aarch64, but Debian calls it arm64, so we need to map
between the two.

See #1973

cmake/CPackConfig.cmake

@@ -109,7 +109,16 @@ if(UNIX AND NOT APPLE)
   set(CPACK_DEB_COMPONENT_INSTALL "ON")
   set(CPACK_DEBIAN_PACKAGE_SHLIBDEPS "ON")
   set(CPACK_DEBIAN_PACKAGE_DEPENDS "fonts-linuxlibertine (>= 5.3.0)")
-  set(CPACK_DEBIAN_PACKAGE_ARCHITECTURE ${CMAKE_SYSTEM_PROCESSOR})
+
+  # Set the correct architecture. Be careful with naming differences between
+  # GCC triplets and Debian
+  set(DEBIAN_ARCHITECTURE_aarch64 arm64)
+  if (DEFINED DEBIAN_ARCHITECTURE_${CMAKE_SYSTEM_PROCESSOR})
+    set(CPACK_DEBIAN_PACKAGE_ARCHITECTURE
+        ${DEBIAN_ARCHITECTURE_${CMAKE_SYSTEM_PROCESSOR}})
+  else()
+    set(CPACK_DEBIAN_PACKAGE_ARCHITECTURE ${CMAKE_SYSTEM_PROCESSOR})
+  endif()
 
 endif()