Follow DW_AT_specification when looking up DW_AT_type attributes.

dwarf_attrval_*() will search the parent DIE referenced by a
DW_AT_abstract_origin attribute for the value of the DW_AT_type attribute.
Do the same thing for the DW_AT_specification attributes in variable
definitions emitted by GCC 6.2, and ensure that we return an error rather
than crashing if neither DW_AT_abstract_origin or DW_AT_specification is
found when looking for the value of the DW_AT_type attribute.

PR: 215350, 215395
Reviewed by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8920