x86: mmu: cast to uintptr_t for page table using Z_X86_PHYS_ADDR

When feeding &z_shared_kernel_page_start directly to
Z_X86_PHYS_ADDR(), the compiler would complain array subscript
out of bound if linking in virtual address space. So cast it
into uintptr_t first before translation.

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