Reviewed by Oliver Hunt.

JavaScriptCore: 

This patch adds a class 'StackBounds', to hold information about the machine stack.
The implementation of this class broadly adheres to the current implmentation of
stack limit checking, and as such does not solve the problem of determining stack
extent, but gives us a common place to do so.

Currently two mechanism are provided to determine the stack origin (the point the
stack is growing away from). currentThreadStackBase() in Collector provides a
more accurate determination of the stack origin, so use this to calculate
StackBounds::m_origin; WTFThreadData::approximatedStackStart is less accurate, and
as such can be removed.  Cache the StackBounds on WTFThreadData such that they
need only be determined once per thread, and for non-API contexts cache this
information in JSGlobalData, to save a thread-specific access.

For the time being retain the estimate of stack size used by JSC's parser
(128 * sizeof(vo...