3.4.0-rc1 is broken due to asm/system.h restructure

The asm/system.h patches missed ARCH=um
which caused build breakage [2].

The commit can be used as an example [2].
I've failed to produce fix myself. Sorry.

[1]:
> commit f05e798ad4c09255f590f5b2c00a7ca6c172f983
> Author: David Howells <dhowells <at> redhat.com>
> Date:   Wed Mar 28 18:11:12 2012 +0100
>
>    Disintegrate asm/system.h for X86

[2]:
linux-2.6/arch/um/drivers/mconsole_kern.c: In function 'stack_proc':
linux-2.6/arch/um/drivers/mconsole_kern.c:707:2: error: implicit declaration of function 'switch_to'
make[2]: *** [arch/um/drivers/mconsole_kern.o] Error 1
make[2]: Target `__build' not remade because of errors.
make[1]: *** [arch/um/drivers] Error 2
  CC      kernel/sys.o
  CC      kernel/sched/core.o
linux-2.6/kernel/sched/core.c: In function 'context_switch':
linux-2.6/kernel/sched/core.c:2086:2: error: expected ':' or ')' before '__percpu_arg'
make[3]: *** [kernel/sched/core.o] Error 1
make[3]: Target `__build' not remade because of errors.
make[2]: *** [kernel/sched] Error 2
make[2]: Target `__build' not remade because of errors.
make[1]: *** [kernel] Error 2
  CC      fs/exec.o
linux-2.6/fs/exec.c: In function 'setup_arg_pages':