Double-hammer: Add "volatile" for reads intended for hammering
Double-hammer: Add "volatile" for reads intended for hammering

Otherwise, a good optimiser could notice that "sum" is never used.

shekkbuilder/rowhammer-test,shekkbuilder/rowhammer-test,kevinmel2000/rowhammer-test,mseaborn/rowhammer-test,kevinmel2000/rowhammer-test,mseaborn/rowhammer-test,kevinmel2000/rowhammer-test,shekkbuilder/rowhammer-test,mseaborn/rowhammer-test,shekkbuilder/rowhammer-test,kevinmel2000/rowhammer-test,mseaborn/rowhammer-test