Fix regression introduced in r328806, preventing boot at least on all
PowerPC Apple hardware, and likely all Open Firmware systems.

The loader would allocate memory for its heap at whatever address Open
Firmware gave it, which would in general be the lowest unallocated address,
usually starting a page or two above 0. As the kernel is linked at 1 MB,
and loader insists on running the kernel at its link address, any heap
larger than 1 MB would overlap the kernel, causing loader memory allocations
to corrupt the kernel and vice versa.

Although r328806 made this problem much worse by increasing the heap size
to 8 MB, causing 88% of the loader heap to overlap with the kernel, the
problem has always existed. The old heap size was 1 MB and, unless that
started exactly at zero, which would cause other problems, some number of
pages of the loader heap still overlapped with the kernel.

This patch solves the issue in two ways and cleans up some related code:

• Moves the loader heap inside of the loader. This guarantees that the heap will be contiguous with the loader and simplifies the heap allocation code at no cost, since the heap lives in BSS.
• Moves the loader, previously at 28 MB and dangerously close to the kernel it loads, a bit higher to 44 MB. This has the effect of breaking loader on non-embedded PPC machines with < 48 MB of RAM, but we did not support those anyway.

The fundamental problem is that the way loader loads ELF files is
incredibly fragile, but that can't be fixed without fundamental
architectural changes.

MFC after: 10 days