fix directory entry in arch-x86-Makefile

Doing a make randconfig I came across this error in the Makefile.

This patch makes a directory out of arch/x86/mach-default for
CONFIG_X86_RDC321X
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/Makefile

@@ -143,7 +143,7 @@ mcore-$(CONFIG_X86_ES7000)	:= arch/x86/mach-default/
 
 # RDC R-321x subarch support
 mflags-$(CONFIG_X86_RDC321X)	:= -Iinclude/asm-x86/mach-rdc321x
-mcore-$(CONFIG_X86_RDC321X)	:= arch/x86/mach-default
+mcore-$(CONFIG_X86_RDC321X)	:= arch/x86/mach-default/
 core-$(CONFIG_X86_RDC321X)	+= arch/x86/mach-rdc321x/
 
 # default subarch .h files