move atomic fields in host
move atomic fields in host

atomic fields need to go at the top of the struct for compatibility with
32bit architectures.

mnsl/Sia,johnathanhowell/Sia,ChrisSchinnerl/Sia,mharkus/Sia,MrStrong/Sia,mnsl/Sia,ericflo/Sia,NebulousLabs/Sia,mtlynch/Sia,rivine/rivine,mtlynch/Sia,ericflo/Sia,rivine/rivine,NebulousLabs/Sia,ChrisSchinnerl/Sia,mharkus/Sia,Mingling94/Sia,MrStrong/Sia