page size in druntime is a mess

[deadalnix]

Page size in druntime is sometime a constant (4Kb), sometime calculated, 
often assumed to be a compile time constant.

druntime should define a proper, authoritative, place to calculate that 
page size, and then use the page calculated here.

I did some tests today, and it require quite a lot of changes. But it is 
mandatory to run D on any system where the page size isn't 4Kb.

So first question, where this should be calculated ? In core.memory ?