unwind: fix unwinding of VDSO frames on i386

May or may not be elfutils bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1129756

Fixes #163.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #176 from abrt/gh163_vdso_workaround

unwind: fix unwinding of VDSO frames on i386