[i386] Add explicit ""aw", @nobits" declarations to stack sections

As reported by Stefan, commit 13d09e6 ("[i386] Simplify linker script
and standardise linker-defined symbol names") breaks gdb, readelf and
associated utilities.

This is caused by the .stack section overwriting a block in the middle
of the .debug_info section (despite being included in the
.bss.textdata section in the output file, which apparently has the
correct attributes for a .bss section).

Fixed by adding explicit flags and type to the stack section
declaration.

src/arch/i386/core/stack.S

  * Internal stack
  ****************************************************************************
  */
-	.section ".stack"
+	.section ".stack", "aw", @nobits
 	.align 8
 	.globl _stack
 _stack:

src/arch/i386/core/stack16.S

  * Internal stack
  ****************************************************************************
  */
-	.section ".stack16"
+	.section ".stack16", "aw", @nobits
 	.align 8
 	.globl _stack16
 _stack16: