libbacktrace: read symbol table of debuginfo file

libbacktrace: read symbol table of debuginfo file

These become another symbol table to search.  This is needed if people
use --strip-all rather than --strip-debug when adding a debuglink
section.

Patch is from GitHub user pickard1.

Fixes https://github.com/ianlancetaylor/libbacktrace/issues/113.

	* elf.c (elf_add): Add the symbol table from a debuginfo file.
	* Makefile.am (MAKETESTS): Add buildidfull and gnudebuglinkfull
	variants of buildid and gnudebuglink tests.
	(%_gnudebuglinkfull, %_buildidfull): New patterns.
	* Makefile.in: Regenerate.