Add relro support

Add support for PT_GNU_RELRO. This allows the static linker to
indicate that certain regions of memory should be marked as
"read-only" after dynamic linking is complete.

See:
  * http://www.akkadia.org/drepper/nonselsec.pdf (section 6)
  * http://tk-blog.blogspot.com/2009/02/relro-not-so-well-known-memory.html

Note that this change has no effect on Android right now, because
we don't compile our code with relro enabled.

Change-Id: I6541f8775367e8558b4388f7d105b1ae6e8f046b

linker/linker.h

@@ -31,7 +31,8 @@
 
 #include <unistd.h>
 #include <sys/types.h>
-#include <linux/elf.h>
+#include <elf.h>
+#include <sys/exec_elf.h>
 
 #undef PAGE_MASK
 #undef PAGE_SIZE
@@ -143,6 +144,10 @@ struct soinfo
     struct link_map linkmap;
 
     int constructors_called;
+
+    Elf32_Addr gnu_relro_start;
+    unsigned gnu_relro_len;
+
 };