libbacktrace: improve XCOFF support

libbacktrace/ChangeLog:
2021-06-28  Clément Chigot  <clement.chigot@atos.net>

	* xcoff.c (SSUBTYP_DWRNGES): New define.
	(xcoff_add): Use correct XCOFF DWARF section subtype
	for DEBUG_RANGES. Remove lineoff workaround.
	Adjust base_address.
	(xcoff_initialize_syminfo): Adapt to new base_address.
	(xcoff_lookup_pc): Likewise.
	(xcoff_initialize_fileline): Likewise.