Fix ia64 build errors (fallout from system.h disintegration)

Fix this build error on ia64:

  In file included from include/linux/sched.h:92,
                  from arch/ia64/kernel/asm-offsets.c:9:
  include/linux/llist.h:59:25: error: asm/cmpxchg.h: No such file or directory
  make[1]: *** [arch/ia64/kernel/asm-offsets.s] Error 1

Right now we don't seem to need any actual contents for the
asm/cmpxchg.h to make the build work ...  so leave the migration of
xchg() and cmpxchg() to this new header file for a future patch.

Also process.c needs <asm/switch_to.h> (for definition of pfm_syst_info).

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/ia64/include/asm/cmpxchg.h

@@ -0,0 +1 @@
+/* Future home of xchg() and cmpxchg() */

arch/ia64/kernel/process.c

@@ -38,6 +38,7 @@
 #include <asm/pgalloc.h>
 #include <asm/processor.h>
 #include <asm/sal.h>
+#include <asm/switch_to.h>
 #include <asm/tlbflush.h>
 #include <asm/uaccess.h>
 #include <asm/unwind.h>