Move boot stack pointer to the top of the page.
x86 stacks grow down, so starting at 0x8000 it was decreasing into the
bootparams page.  Give the stack its own page and start at the top of
it.
// The 'zero page', a.k.a linux kernel bootparams.
pub const ZERO_PAGE_START: usize = 0x7000;
// Initial stack for the boot CPU.
-pub const BOOT_STACK_POINTER: usize = 0x8000;
+pub const BOOT_STACK_START: usize = 0x8000;
+pub const BOOT_STACK_POINTER: usize = 0x8ff0;
// Initial pagetables.
pub const PML4_START: usize = 0x9000;
pub const PDPTE_START: usize = 0xa000;
