To simplify polling code and to be able to preserve ordering between handshakes
we should implement safepoints with handshakes.
The polling code special handles safepoints and could smaller and simpler if the
call to SafepointSynchronize::block was inside a handshake operation.
After 8238761: Asynchronous handshakes, when there is per JavaThread handshake
operation queue preserving ordering of the handshakes and safepoints would be
automagic.