sh/jdk build failure on Windows

[Aleksey Shipilev]

I believe recent pushes to sh/jdk broken Windows builds:

c:\cygwin64\home\windows\worker\shenandoah-jdkX-windows\build\src\hotspot\share\gc/shenandoah/shenandoahLock.hpp(72)
: error C2653: 'SafepointSynchronize' : is not a class or namespace name
c:\cygwin64\home\windows\worker\shenandoah-jdkX-windows\build\src\hotspot\share\gc/shenandoah/shenandoahLock.hpp(72)
: error C3861: 'is_at_safepoint': identifier not found

That probably only manifests without precompiled headers. Zhengyu, can you take a look?

-- 
Thanks,
-Aleksey