libdw: Make dwarf_srclang forward compatible with DW_AT_language_name

libdw: Make dwarf_srclang forward compatible with DW_AT_language_name

For programs which use dwarf_srclang it might be useful to translate a
DW_LNAME (plus version) into a DW_LANG constant if there is no
DW_AT_language constant, but there is a DW_AT_language_name (and
DW_AT_language_version).

	* libdw/dwarf_srclang.c (language_to_srclang): New function.
	(dwarf_srclang): If there is no DW_AT_language, try
	DW_AT_language_name and DW_AT_language_version, use
	language_to_srclang.

Signed-off-by: Mark Wielaard <mark@klomp.org>