tile: fix multiple build failures from system.h dismantle

Commit bd119c69239322caafdb64517a806037d0d0c70a

    "Disintegrate asm/system.h for Tile"

created the asm/switch_to.h file, but did not add an include
of it to all its users.

Also, commit b4816afa3986704d1404fc48e931da5135820472

        "Move the asm-generic/system.h xchg() implementation to asm-generic/cmpxchg.h"

introduced the concept of asm/cmpxchg.h but the tile arch
never got one.  Fork the cmpxchg content out of the asm/atomic.h
file to create one.

Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
index 30caeca..ee01b1c 100644
--- a/arch/tile/kernel/process.c
+++ b/arch/tile/kernel/process.c
@@ -28,6 +28,7 @@
 #include <linux/tracehook.h>
 #include <linux/signal.h>
 #include <asm/stack.h>
+#include <asm/switch_to.h>
 #include <asm/homecache.h>
 #include <asm/syscalls.h>
 #include <asm/traps.h>