Introduce TARGET fallbacks in CMake

X86(-64) targets for cross-compilation are likely to be for a simulator/testing and not a production environment. Regardless, we do not know information about the target yet, so choose a default of core2/penryn.

cmake/system.cmake

@@ -13,6 +13,10 @@ if(CMAKE_CROSSCOMPILING AND NOT DEFINED TARGET)
     set(TARGET "ARMV8")
   elseif(ARM)
     set(TARGET "ARMV7") # TODO: Ask compiler which arch this is
+  elseif(X86_64)
+    set(TARGET "PENRYN")
+  elseif(X86)
+    set(TARGET "CORE2")
   else()
     message(FATAL_ERROR "When cross compiling, a TARGET is required.")
   endif()