bsps/powerpc: Move .rtemstack section

Move the .rtemsstack section from a read-only to a read-write area, see
page table setup in __BSP_default_pgtbl_setup().

bsps/powerpc/shared/start/linkcmds.share

@@ -97,10 +97,6 @@ SECTIONS
   _TLS_Size = _TLS_BSS_end - _TLS_Data_begin;
   _TLS_Alignment = MAX (ALIGNOF (.tdata), ALIGNOF (.tbss));
 
-  .rtemsstack (NOLOAD) : {
-    *(SORT(.rtemsstack.*))
-  } >CODE
-
   /* Adjust the address for the data segment.  We want to adjust up to
      the same address within the page on the next page up.  */
   . = ALIGN(0x10000) + (. & (0x10000 - 1));
@@ -245,6 +241,9 @@ SECTIONS
   . = ALIGN(16);
    PROVIDE (__bss_end = .);
   } > CODE
+  .rtemsstack (NOLOAD) : {
+    *(SORT(.rtemsstack.*))
+  } >CODE
   . = ALIGN(16);
   _end = . ;
   __rtems_end = . ;