Fence: Change AC_FENCE_ACTION to an enum

[muramura]

Change AC_FENCE_ACTION to an ENUM. Using an ENUM allows for grouping actions. I believe using an ENUM is better than indicating groups with define names.

[Ryanf55]

I like this kind of PR. Any motive for mixing of c-style cast and static cast?

[muramura]

@Ryanf55 san. I changed it to static_cast.

[peterbarker]

Ping @muramura don't forget this one