[libbacktrace] Add altlink field to struct dwarf_data

[libbacktrace] Add altlink field to struct dwarf_data

Add an altlink field to struct dwarf_data, and initialize it with the pointer
to the struct dwarf_data for the .gnu_debugaltlink.

2019-01-17  Tom de Vries  <tdevries@suse.de>

	* dwarf.c (struct dwarf_data): Add altlink field.
	(backtrace_dwarf_add): Add and handle fileline_altlink parameter.
	* elf.c	(elf_add): Add argument to backtrace_dwarf_add call.
	(phdr_callback, backtrace_initialize): Add argument to elf_add calls.
	* internal.h (backtrace_dwarf_add): Add fileline_altlink parameter.
	* pecoff.c (coff_add): Add argument to backtrace_dwarf_add call.
	* xcoff.c (xcoff_add): Same.

From-SVN: r267994