libdw: Add support for reading DW_FORM_addrx[1234] in .debug_addr.

libdw: Add support for reading DW_FORM_addrx[1234] in .debug_addr.

Recognize the new .debug_addr section. The CU will now hold a new
address base offset in that section for that CU. dwarf_form_addr will
decode DW_FORM_addrx[1234] and return addresses using that address
base from the .debug_addr. A new internal function read_3ubyte_unaligned
will try to read a 24-bit value depending on endianness of the underlying
file.

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