libdw: dwarf_aggregate_size() works with multi-dimensional arrays

libdw: dwarf_aggregate_size() works with multi-dimensional arrays

If we have a multidimensional array of dimensions (a,b,c) the number of elements
should be a*b*c, but prior to this patch dwarf_aggregate_size() would report
a+b+c instead.

This patch fixes the bug and adds a test that demonstrates the bug (the test
fails without the functional part of this patch).

Fixes: https://sourceware.org/bugzilla/show_bug.cgi?id=22546
Signed-off-by: Dima Kogan <dima@secretsauce.net>