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

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).

---

new file mode 100644
index 0000000..507c66c
@@ -0,0 +1 @@
+/* Future home of xchg() and cmpxchg() */

index 9dc52b6..ce74e14 100644
@@ -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>