v68k-mac:  Fix reading mutable low memory globals

This was completely broken and couldn't possibly have done anything with
any resemblence to working except by coincidence.
v68k-alloc:  Move constants into header

Having publically visible constants allows range and alignment checks to
be inlined and optimized away.
v68k-alloc:  allocate() for existing native memory

Factor out new function allocate_n_pages_for_existing_alloc_unchecked(),
which takes a page count and a pointer to a native memory block matching
in size.

Add an inline two-arg allocate() that takes a native pointer plus a byte
count and calls allocate_n_pages_for_existing_alloc_unchecked() -- after
some computation which can be optimized away when the value of 'size' is
known at compile-time.  The size of the memory must be a multiple of the
page size, because the allocator just uses whole pages and doesn't store
the actual allocation size anywhere, so an entire page must be backed by
real memory.
v68k-alloc:  Factor out deallocate_existing()

In deallocate_existing(), return the native memory allocation instead of
freeing it.
v68k-alloc:  Factor out allocate_n_pages()

A simple wrapper of allocate_n_pages_for_existing_alloc_unchecked(), new
function allocate_n_pages() first compares the page count to the maximum
number of pages.
Merge branch 'allocate_n_pages'