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
diff --git a/libc/include/sys/exec_elf.h b/libc/include/sys/exec_elf.h
index 03bbdac..eecbd64 100644
--- a/libc/include/sys/exec_elf.h
+++ b/libc/include/sys/exec_elf.h
@@ -130,4 +130,6 @@
 #define STT_LOPROC	13		/* reserved range for processor */
 #define STT_HIPROC	15		/*  specific symbol types */
 
+#define PT_GNU_RELRO    0x6474e552      /* Read-only post relocation */
+
 #endif /* _SYS_EXEC_ELF_H_ */