libdw: Add new DWARF5 Dwarf expression operations.

libdw: Add new DWARF5 Dwarf expression operations.

DW_OP_implicit_pointer, DW_OP_entry_value, DW_OP_const_type,
DW_OP_regval_type, DW_OP_deref_type, DW_OP_xderef_type,
DW_OP_convert and OP_reinterpret are implemented like their
pre-DWARF5 GNU variants.

DW_OP_xderef_type is implemented as a (non-CU relative) variant of
DW_OP_deref_type.

DW_OP_addrx and DW_OP_constx are recognized but not interpreted yet.

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