x86: gen_mmu: also map SRAM if linking in virtual address space

When linking in virtual address space, we still need physical
addressed in SRAM to be mapped so platform can boot from physical
memory and to access structure necessary for boot (e.g. GDT and
IDT). So identity maps the kernel in SRAM.

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