targets: explicitly mark the stack as NOLOAD

This prevents it from being of type PROGBITS in lld 9, it should always
be NOBITS. It should fix the following error in lld 9:

    ROM segments are non-contiguous

targets/avr.ld

@@ -17,7 +17,7 @@ SECTIONS
         *(.rodata.*)
     }
 
-    .stack :
+    .stack (NOLOAD) :
     {
         . += _stack_size;
         _stack_top = .;