boards: x86: enable MULTIBOOT_MEMMAP

QEMU provides multiboot information by default so we can
use the provided memory map to mark reserved physical
memory. Note that 64-bit requires Multiboot2 which
currently both Zephyr and QEMU do not support, hence
it's not enabled for qemu_x86_64.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>