newlib: memory-map the heap, cleanups

We now draw heap memory from an anonymous memory mapping
instead of a hard-coded region past the kernel image,
which is no longer mapped by default.

Some readability cleanups were made to a particuarly
horrible set of nested ifdefs. A few types were adjusted.

sbrk()'s count argument is an intptr_t, not an int.

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