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: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>