efi/libstub: Fix gcc error around __umoddi3 for 32 bit builds

32bit gcc doesn't support modulo operation on 64 bit data. It results in
a __umoddi3 error while building EFI for 32 bit.

Use bitwise operations instead of modulo operations to fix the issue.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Link: https://lore.kernel.org/r/20200625234516.31406-2-atish.patra@wdc.com


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