libdw: Try both the relative and absolute paths when finding a .dwo file.

We would give up if one of them failed. With this fixed a self-test with
make check succeeds when building elfutils itself with CFLAGS set to
"-gdwarf-4 -gdwarf-split -O2".

Signed-off-by: Mark Wielaard <mark@klomp.org>
4 files changed