veyron/runtimes/google/rt: Fix a race condition in our signal handling.
veyron/runtimes/google/rt: Fix a race condition in our signal handling.

I had previously removed signals as a sync.Once initialized global variable, but
I failed to actually remove the global variable.  This caused a race to
happen on that value.

Change-Id: Iace5cdd6b08cedcfa8db5981f8aa8381d45387ea

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref