x86/efistub: Revert to heap allocated boot_params for PE entrypoint

This is a partial revert of commit

  8117961d98f ("x86/efi: Disregard setup header of loaded image")

which triggers boot issues on older Dell laptops. As it turns out,
switching back to a heap allocation for the struct boot_params
constructed by the EFI stub works around this, even though it is unclear
why.

Cc: Christian Heusel <christian@heusel.eu>
Reported-by:  <mavrix#kernel@simplelogin.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>