The linked.ld script is extended to include the _kernel_start and
_kernel_end symbols which are passed by the rt0 code to Kmain. The
allocator converts these addresses to a start/end frame index by
rounding down the kernel start address to the nearest page and rounding
up the kernel end address to the nearest page.
When allocating frames, the allocator will treat the region defined by
these 2 indices as reserved and skip over it.
AllocFrame now rounds up the region start address to the nearest page
multiple and rounds down the region end address to the nearest page
multiple. It also ignores memory regions with size smaller than a page.
Instead of using frame indices and converting them to a pmm.Frame, the
allocator now just keeps track of the last allocated pmm.Frame.
As the allocator is now unexported, a package-exported Init() method is
now provided whose purpose is to initialize the physical allocator
sub-system.
