The error in the GDT caused the whole memory space to be offset by
128B or 256B (It's not entirely sure which one of those). This was
only made visible by later changes, but now it's fixed.
This commit deletes the gdt.asm for the i386 architecture as it was
quite buggy in that state. In a short while, the real GDT code which
works is going to be committed.
This commit adds a check for whether the A20 line is usable; checking
whether more than one megabyte of RAM can be used. If it isn't usable,
it currently isn't activated, but rather, the boot process is stopped.
Eric-Paul Ickhorn