gas: Emit name, comp_dir and producer strings in .debug_str.

gas: Emit name, comp_dir and producer strings in .debug_str.

Putting the name, comp_dir and producer strings in the .debug_str section
makes it possible to share them across CUs. This saves a small amount of
space (about ~20K on a glibc libc.so.6 build with debuginfo). And makes
it easier for tools like rpm debugedit to adjust the source paths when
generating separate debuginfo files.

gas/
       * dwarf2dbg.c (out_debug_abbrev): Use DW_FORM_strp instead of
       DW_FORM_string for DW_AT_name, DW_AT_comp_dir and DW_AT_producer.
       (out_debug_info): Accept symbols to name, comp_dir and producer
       in the .debug_str section and emit those offsets not full strings.
       (out_debug_str): New function that outputs the strings for name,
       comp_dir and producer in .debug_str and generates symbols to
       those strings.
       (out_debug_line): Create a .debug_str section if necessary and
       call out_debug_str before calling out_debug_info.
       * testsuite/gas/aarch64/dwarf.d: Add extra section symbol to
       expected output.