libdwfl: specify optional sysroot to search for shared libraries and binaries

libdwfl: specify optional sysroot to search for shared libraries and binaries

When searching the list of modules in a core file, if the core was
generated on a different system to the current one, we need to look
in a sysroot for the various shared objects.

For example, we might be looking at a core file from an ARM system
using elfutils running on an x86 host.

This change adds a new function, dwfl_set_sysroot(), which then
gets used when searching for libraries and binaries.

Signed-off-by: Luke Diamand <ldiamand@roku.com>
Signed-off-by: Michal Sekletar <msekleta@redhat.com>