nacl_helper_bootstrap: associate .rodata with :text PHDR, fix build on GCC

Author: illwieckz

src/trusted/service_runtime/linux/nacl_bootstrap.x

@@ -78,7 +78,7 @@ SECTIONS {
   .rodata : {
     *(.rodata*)
     *(.eh_frame*)
-  }
+  } :text
 
   etext = .;