s390/switch_to: use generic header file

Move the switch_to() implementation to process.c and use the generic
switch_to.h header file instead, like some other architectures.

This addresses also the oddity that the old switch_to() implementation
assigns the return value of __switch_to() to 'prev' instead of 'last',
like it should.

Remove also all includes of switch_to.h from C files, except process.c.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/stacktrace.h

@@ -4,7 +4,6 @@
 
 #include <linux/uaccess.h>
 #include <linux/ptrace.h>
-#include <asm/switch_to.h>
 
 struct stack_frame_user {
 	unsigned long back_chain;