Don't expose guard pages to malloc_stack API consumers (#54591)
Whether or not a guard page is in effect is an implementation detail and
consumers of the `malloc_stack` API should not have to worry about that.
In particular, if a stack of a certain size is requested, a stack of that
size should be delivered, and not be reduced on some systems because we
park a guard page in that range.
This also helps consumers of the gcext API implementing stack scanning
(i.e., GAP.jl), as it does not have to worry about running into those
guard pages anymore.
