kernel/x86_64: setup a new team in compatibility mode.

* in load_image_internal(), elf32_load_user_image checks whether the binary
format requires the compatibility mode.
* we then set up the flag THREAD_FLAGS_COMPAT_MODE and the address space size.
* the compatibility mode runtime_loader is hardcoded with x86/runtime_loader.
* if needed, the 64-bit flat_args structure is converted in-place to its 32-bit
layout.
* a 32-bit flat_args isn't handled yet (a 32-bit team execs a 64-bit binary).

Change-Id: Ia6a066bde8d1774d85de29b48dc500e27ae9668f

headers/private/system/thread_defs.h

@@ -37,6 +37,10 @@ enum {
 #define THREAD_CREATION_FLAG_DEFER_SIGNALS	0x01
 	// create the thread with signals deferred, i.e. with
 	// user_thread::defer_signals set to 1
+#ifdef _COMPAT_MODE
+#	define THREAD_CREATION_FLAG_COMPAT_MODE	0x02
+	// create the thread with a compatibility mode
+#endif
 
 
 struct thread_creation_attributes {