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

arch/x86/Makefile

@@ -151,7 +151,6 @@ mflags-y += -Iinclude/asm-x86/mach-default
 # 64 bit does not support subarch support - clear sub arch variables
 fcore-$(CONFIG_X86_64)  :=
 mcore-$(CONFIG_X86_64)  :=
-mflags-$(CONFIG_X86_64) :=
 
 KBUILD_CFLAGS += $(mflags-y)
 KBUILD_AFLAGS += $(mflags-y)