All archs use the same cpu_loop_exit, so move the prototype in a common
header. i386 was carrying a __hidden attribute, but that was empty for
this arch anyway.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5820 c046a42c-6fe2-441c-8c8c-71466251a162