[arch][mmu] clean up page size definitions in each arch's defines.h

No real functional change except how the smaller ARCH_DEFAULT_PAGE_SIZE
is now computed and set in defines.h instead of rules.mk for arch/arm to
be consistent with the other arch that has a large/small build (riscv).

arch/m68k/include/arch/defines.h

@@ -7,10 +7,10 @@
  */
 #pragma once
 
-#define PAGE_SIZE 4096
 #define PAGE_SIZE_SHIFT 12
+#define PAGE_SIZE       (1U << PAGE_SIZE_SHIFT)
 
 // XXX is this right?
 #define CACHE_LINE 32
 
-#define ARCH_DEFAULT_STACK_SIZE 4096
+#define ARCH_DEFAULT_STACK_SIZE PAGE_SIZE