zipl/boot: declare that no executable stack is required

Normally, `gcc` takes care of adding the `GNU_STACK` marking except for
assembly code. Therefore, let's add the marking manually. Discard the
`.note.GNU-stack` section in the linker script since it's just a
"message" from the compiler to the linker. This fixes the linker
warning:

ld: warning: entry.o: missing .note.GNU-stack section implies executable stack

Acked-by: Eduard Shishkin <edward6@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

zipl/boot/stage3.lds.S

@@ -79,5 +79,6 @@ SECTIONS
   /DISCARD/ : {
     *(.eh_frame)
     *(.interp)
+    *(.note.GNU-stack)
   }
 }