Let runtime_loader ignore RT_RELRO program headers.

* The new libstdc++.so contains program headers of type PT_RELRO (for
  making segments read-only after relocation). While the actual feature
  has not been implemented, the runtime_loader should now silently
  accept (and ignore) those program headers.

headers/private/system/elf_common.h

@@ -141,6 +141,7 @@
 #define PT_PHDR		6
 #define PT_TLS		7
 #define PT_STACK	0x6474e551
+#define PT_RELRO	0x6474e552
 
 #define PT_LOPROC	0x70000000
 #define PT_HIPROC	0x7fffffff