Zero %ebp and %edi when entering Linux's 32-bit entry point, as

Zero %ebp and %edi when entering Linux's 32-bit entry point, as
required by the boot protocol.
* include/grub/i386/relocator.h (struct grub_relocator32_state): Add
ebp and edi members.
* grub-core/lib/i386/relocator.c (grub_relocator_boot): Handle
state.ebp and state.edi.
* grub-core/lib/i386/relocator32.S (grub_relocator32_start): Set
%ebp and %edi according to grub_relocator32_ebp and
grub_relocator32_edi respectively.
* grub-core/loader/i386/linux.c (grub_linux_boot): Zero state.ebp
and state.edi.