This patch moves details about the stack allocation and organization
the base-internal headers. Thereby, I replaced the notion of "thread
contexts" by "stacks" as this term is much more intuitive. The fact that
we place thread-specific information at the bottom of the stack is not
worth introducing new terminology.
Issue #1832
On seL4 and L4/Fiasco, we employ a simple yielding spinlock as lock
implementation. Consequently these base platforms used to have a
simplified header. However, since the regular cancelable_lock has all
the member variables needed to implement a spinlock, we can simply use
the generic header on those two platforms too, just leaving some other
parts of the generic header unused. So at API level, the difference is
not visible.
Issue #1832
