asm/system.h: um: arch_align_stack() moved to asm/exec.h

arch_align_stack() moved to asm/exec.h, so change the comment referring to
asm/system.h which no longer exists.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/kernel/process.c

@@ -359,7 +359,7 @@ int singlestepping(void * t)
 /*
  * Only x86 and x86_64 have an arch_align_stack().
  * All other arches have "#define arch_align_stack(x) (x)"
- * in their asm/system.h
+ * in their asm/exec.h
  * As this is included in UML from asm-um/system-generic.h,
  * we can use it to behave as the subarch does.
  */