efi: Remove arch specific image headers for RISC-V, ARM64 and ARM

efi: Remove arch specific image headers for RISC-V, ARM64 and ARM

The arch specific image header details are not very useful as most of
the GRUB just looks at the PE/COFF spec parameters (PE32 magic and
header offset).

Remove the arch specific images headers and define a generic arch
headers that provide enough PE/COFF fields for the GRUB to parse
kernel images correctly.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>