Kernel: Use the new on demand ELF structure

All executable files are now read from disk and paged on demand.
This was a big rewrite of the old ELF library but in the end
everything seems much cleaner, since all the old functionality was
not actually needed for execution.

I have to do some measurements, but I feel like memory usage dropped
quite a bit after this change.

kernel/CMakeLists.txt

@@ -118,7 +118,7 @@ set(LIBC_SOURCES
 )
 
 set(LIBELF_SOURCES
-	../LibELF/LibELF/ELF.cpp
+	../LibELF/LibELF/LoadableELF.cpp
 )
 
 set(KERNEL_SOURCES