arch/x86: Map 512G by default when booting x86_64

The default page table (the architecturally required one used for
entrance to long mode, before the OS page tables get assembled) was
mapping the first 4G of memory.

Extend this to 512G by fully populating the second level page table.
We have devices now (up_squared) which have real RAM mapped above 4G.
There's really no good reason not to do this, the page is present
always anyway.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>