Link with mixed IR/non-IR objects

* 2 kinds of object files
  o non-IR object file has
    * non-IR sections
  o IR object file has
    * IR sections
    * non-IR sections
    * The output of "ld -r" with mixed IR/non-IR objects should work with:
        o Compilers/linkers with IR support.
	o Compilers/linkers without IR support.
* Add the mixed object file which has
  o IR sections
  o non-IR sections:
    * Object codes from IR sections.
    * Object codes from non-IR object files.
  o Object-only section:
    * With section name ".gnu_object_only" and SHT_GNU_OBJECT_ONLY
    (0x6ffffff8) type on ELF. 
    * Contain non-IR object file.
    * Input is discarded after link.
* Linker action:
  o Classify each input object file:
    * If there is a ".gnu_object_only" section, it is a mixed object file.
    * If there is a IR section, it is an IR object file.
    * Otherwise, it is a non-IR object file.
  o Relocatable non-IR link:
    * Prepare for an object-only output.
    * Prepare for a regular output.
    * For each mixed object file:
      * Add IR and non-IR sections to the regular output.
      * For object-only section:
      * Extract object only file.
      * Add it to the object-only output.
      * Discard object-only section.
    * For each IR object file:
      * Add IR and non-IR sections to the regular output.
    * For each non-IR object file:
      * Add non-IR sections to the regular output.
      * Add non-IR sections to the object-only output.
    * Final output:
      * If there are IR objects, non-IR objects and the object-only
      output isn't empty:
	* Put the object-only output into the object-only section.
	* Add the object-only section to the regular output.
	* Remove the object-only output.
  o Normal link and relocatable IR link:
    * Prepare for output.
    * IR link:
      * For each mixed object file:
	* Compile and add IR sections to the output.
	* Discard non-IR sections.
	* Object-only section:
	  * Extract object only file.
	  * Add it to the output.
	  * Discard object-only section.
      * For each IR object file:
        * Compile and add IR sections to the output.
	* Discard non-IR sections.
      * For each non-IR object file:
	* Add non-IR sections to the output.
    * Non-IR link:
      * For each mixed object file:
	* Add non-IR sections to the output.
	* Discard IR sections and object-only section.
      * For each IR object file:
	* Add non-IR sections to the output.
	* Discard IR sections.
      * For each non-IR object file:
	* Add non-IR sections to the output.