libdw, libdwfl: Save original path of ELF file

libdw and libdwfl currently save the path of the directory containing
the ELF file to use when searching for alt and dwo files.  To search for
dwp files, we need the file name too.  Add an elfpath field to Dwarf,
and set the debugdir field from it.  Also update libdwfl to set elfpath
and debugdir.

Signed-off-by: Omar Sandoval <osandov@fb.com>
9 files changed