x86: add subarch support (for headers) to x86_64

this patch allows x86_64 to use subarch mach_ headers
in practice, since x86_64 does not have any subarch, it
will use mach_default. But it will allow for substantially
less code duplication

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>