x86/boot/compressed: Move bootargs parsing out of 32-bit startup code

Move the logic that chooses between the different EFI entrypoints out of
the 32-bit boot path, and into a 64-bit helper that can perform the same
task much more cleanly. While at it, document the mixed mode boot flow
in a code comment.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-4-ardb@kernel.org