Make Label compose its associated offset.

	* src/inst/label.hh: Protect default constructor. Compose its
	associated offset. Add a constructor that waits for it.
	* src/inst/section.hh (has_label, get_offset): Update accordingly.
	(register_label): Wait for a Label only and use it to get its offset.
	* src/inst/data_section.hh: Update accordingly.
	* src/inst/text_label.hh: New.
	* src/inst/Makefile.am: Update accordingly.
	* src/inst/text_section.hh: Create TextLabels instead of Labels.
	* src/inst/text_section.cc: Include inst.hh to gain access
	to Inst's interface.