libdw: handle DW_FORM_indirect when reading attributes

libdw: handle DW_FORM_indirect when reading attributes

Whenever we encounter an attribute with DW_FORM_indirect, we need to
read its true form from the DIE data. Then, we can continue normally.
This adds support to the most obvious places: __libdw_find_attr() and
dwarf_getattrs(). There may be more places that need to be updated.

I encountered this when inspecting a file that was processed by our BOLT
tool: https://github.com/facebookincubator/BOLT. This also adds a couple
of test cases using a file generated by that tool.

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