_Actually_ move the kernel to the last TiB.

More work on process page tables, including only mapping the last 2 pml4
entries (the highest 1TiB of the address space, ie, kernel space) into a
new table.

Includes the work of actually moving the kernel there, which I had
apparently done in name only previously. Oops.

src/kernel/gdt.cpp

@@ -113,6 +113,7 @@ tss_set_entry(uint8_t i, uint64_t base, uint64_t limit)
 	tssd.base_16 = (base >> 16) & 0xff;
 	tssd.base_24 = (base >> 24) & 0xff;
 	tssd.base_32 = (base >> 32) & 0xffffffff;
+	tssd.reserved = 0;
 
 	tssd.type =
 		gdt_type::accessed |