The Secondary allocator wraps the platform allocation primitives. It is
meant to be used for larger sizes that the Primary can't fullfill, as
it will be slower, and sizes are multiple of the system page size.
This also changes some of the existing code, notably the opaque
platform data being passed to the platform specific functions: we can
shave a couple of syscalls on Fuchsia by storing additional data (this
addresses a TODO).