x86: intel64: set up memory sections

Some linker symbols indicating the bounds of various memory
areas needed for setting up boot MMU regions are now in
place.

MMU_BOOT_REGION macros added to align the bounds of these
sections if we're using runtime page tables.

_image_rodata_start moved to the proper place, it was not
accounting for the rodata sections specified in
linker/common_rom.ld.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>