Compile qemu-timer only once

Arrange various declarations so that also non-CPU code can access
them, adjust users.

Move CPU specific code to cpus.c.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/exec.c b/exec.c
index 0916208..8274ec2 100644
--- a/exec.c
+++ b/exec.c
@@ -38,6 +38,7 @@
 #include "hw/hw.h"
 #include "osdep.h"
 #include "kvm.h"
+#include "qemu-timer.h"
 #if defined(CONFIG_USER_ONLY)
 #include <qemu.h>
 #include <signal.h>