[gdb/contrib] Fix errno.EOPNOTSUP in dwarf-to-dwarf-assembler.py

When running dwarf-to-dwarf-assembler.py without arguments, I run into:
...
$ ./gdb/contrib/dwarf-to-dwarf-assembler.py
Usage:
python ./asm_to_dwarf_assembler.py <path/to/elf/file>
Traceback (most recent call last):
  File "/data/vries/gdb/binutils-gdb.git/./gdb/contrib/dwarf-to-dwarf-assembler.py", line 621, in main
    filename = argv[1]
               ~~~~^^^
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/data/vries/gdb/binutils-gdb.git/./gdb/contrib/dwarf-to-dwarf-assembler.py", line 642, in <module>
    main(sys.argv)
    ~~~~^^^^^^^^^^
  File "/data/vries/gdb/binutils-gdb.git/./gdb/contrib/dwarf-to-dwarf-assembler.py", line 625, in main
    sys.exit(errno.EOPNOTSUP)
             ^^^^^^^^^^^^^^^
AttributeError: module 'errno' has no attribute 'EOPNOTSUP'. Did you mean: 'EOPNOTSUPP'?
...

Fix this by using errno.EOPNOTSUPP.