goldfish_sync: provide a signal to detect reboot

bug: 31498581

This CL adds a "SYNC_REG_INIT" register to detect reboot.
It is to be submitted together with androidemu and qemu2 changes
that actually detect that register.

Change-Id: I20ae355362824508aa86167aea753aef44c866c2
Signed-off-by: Lingfeng Yang <lfy@google.com>
(cherry picked from commit 3122672e)
(cherry picked from commit b44d486d)