[libbacktrace] Return struct dwarf_data pointer from elf_add

[libbacktrace] Return struct dwarf_data pointer from elf_add

Allow the caller of elf_add access to the struct dwarf_data pointer
corresponding to the added elf.

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

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

From-SVN: r267993