Fix discrepancies in nm's --line-number output by adding support for the DW_AT_specif...

Fix discrepancies in nm's --line-number output by adding support for the DW_AT_specification DWARF Attttribute.

	PR 25676
bfd	* dwarf2.c (struct varinfo): Add unit_offset field to record the
	location of the varinfo in the unit's debug info data.  Change the
	type of the stack field to a boolean.
	(lookup_var_by_offset): New function.  Returns the varinfo
	structure for the variable described at the given offset in the
	unit's debug info.
	(scan_unit_for_symbols): Add support for variables which have the
	DW_AT_specification attribute.

binutils* testsuite/binutils-all/dw4.s: New test source file.
	* testsuite/binutils-all/nm.exp: Run the new test.