newlib: clamp max heap size on MMU systems

Previously, newlib claimed all free physical memory in the
system.

Now, the kernel manages this, allowing for memory to be
used via k_mem_map() calls.

Establish an upper bound to how much newlib will try to
claim on system startup, instead of trying to take all
of it, allowing other parts of the system to also map
anonymous memory.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>