[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).

top/main.c

@@ -23,7 +23,6 @@
 #include <lk/debug.h>
 #include <lk/init.h>
 #include <platform.h>
-#include <string.h>
 #include <target.h>
 
 /* saved boot arguments from whoever loaded the system */