libdw: Handle DW_OP_GNU_variable_value.

libdw: Handle DW_OP_GNU_variable_value.

Handle DW_OP_GNU_variable_value in dwarf_getlocation[_attr,_die].
DW_OP_GNU_variable_value takes one argument a DIE reference that
describes a value given by a location of const_value attribute.

To test handling of the new operand the varlocs test is adapted
to print out all DIEs and attributes with expressions or location
lists (the original varlocs test only prints out variables and
arguments of function DIEs).

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