x86: use CONFIG_SRAM_OFFSET instead of CONFIG_X86_KERNEL_OFFSET

This changes x86 to use CONFIG_SRAM_OFFSET instead of
arch-specific CONFIG_X86_KERNEL_OFFSET. This allows the common
MMU macro Z_BOOT_VIRT_TO_PHYS() and Z_BOOT_PHYS_TO_VIRT() to
function properly if we ever need to map the kernel into
virtual address space that does not have the same starting
physical address.

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