Unmap efiloader remnants once we're in the kernel

[IsaacWoods]

We need to map loader code and data into the kernel's page tables in efiloader as we jump into the kernel, so we can continue fetching code once we switch to them, but before we jump into the kernel. However, it's not great to keep it lingering around for no reason, as we want to reclaim the physical memory.

• Pass sections that remain mapped in the memory map
• Mark them as free in the physical memory manager
• Unmap them from the kernel's page tables