Reformat experimental header for compatibility with old DWARF consumers.

Reformat experimental header for compatibility with old DWARF consumers.

Old consumers will now see a DWARF-4 header, with new fields added at the
end of the header. The new fields and the entire logicals and actuals
tables are wrapped in a single extended opcode so that old consumers
will see just the single opcode.

2015-01-28  Cary Coutant  <ccoutant@google.com>

gas/
	* dwarf2dbg.c (line_base): New variable.
	(line_range): New variable.
	(DWARF2_EXPERIMENTAL_LINE_OPCODE_BASE): Rename.
	(DWARF2_LINE_BASE): Restore to original value.
	(DWARF2_LINE_RANGE): Likewise.
	(DWARF5_EXPERIMENTAL_LINE_BASE): New macro.
	(DWARF5_EXPERIMENTAL_LINE_RANGE): New macro.
	(SPECIAL_LINE): Use line_base, line_range.
	(size_inc_line_addr): Likewise.
	(emit_inc_line_addr): Likewise.
	(out_debug_line): Reformat header for compatibility with old DWARF
	consumers.