This patch moves the switch_to() macro to system.h

As those macros are fundamentally different between i386 and x86_64,
they are enclosed around an ifdef.
Signed-off-by: NGlauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>