efi/libstub: Add a helper function to split 64-bit values

In several places 64-bit values need to be split up into two 32-bit
fields, in order to be backward-compatible with the old 32-bit ABIs.

Instead of open-coding this, add a helper function to set a 64-bit value
as two 32-bit fields.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200430182843.2510180-3-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>