Allow a compilation without -mcmodel=large
* kern/efi/mm.c (grub_efi_allocate_pages): don't allocate >4GiB
when compiled without -mcmodel=large
(filter_memory_map): remove memory post 4 GiB when compiled
without -mcmodel=large
* configure.ac: fail gracefully and add -DMCMODEL_SMALL=1 to
TARGET_CFLAGS when -mcmodel=large isn't supported