libdw: Handle all address FORMs for dwarf_highpc, handle errors better.

libdw: Handle all address FORMs for dwarf_highpc, handle errors better.

dwarf_highpc can use any address FORM, not just DW_FORM_addr. Just try
whether the address can be resolved as address. Always set error when
attribute couldn't be found or resolved. When calculating the base
address for a CU don't try to second guess the error code, just treat
an error the same as the attribute not being there.

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