arch: Simplify handling of the stack and the heap

The stack/heap separation has been removed, allowing us to treat the
stack and the heap as a single block of memory. This allows us to grow
them towards each other rather than giving them both a fixed, non-
overlapping size.

Change-Id: I537535eeb21c947dc657daaacb75ebf2d9eeb7c5
Signed-off-by: Chris Kay <chris.kay@arm.com>