efi/x86: Don't relocate the kernel unless necessary

Add alignment slack to the PE image size, so that we can realign the
decompression buffer within the space allocated for the image.

Only relocate the kernel if it has been loaded at an unsuitable address:

 - Below LOAD_PHYSICAL_ADDR, or
 - Above 64T for 64-bit and 512MiB for 32-bit

For 32-bit, the upper limit is conservative, but the exact limit can be
difficult to calculate.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200303221205.4048668-6-nivedita@alum.mit.edu
Link: https://lore.kernel.org/r/20200308080859.21568-20-ardb@kernel.org