[llvm-ar] Enforce one-dash form for long options

binutils ar does not support one-dash long options (they conflict with
operation code and modifier flags).

```
% ar -help
ar: invalid option -- 'e'
...
% ar -version
ar: invalid option -- 'e'
...

% ar x --plugin=xx x.a  # ok
% ar x -plugin=xx x.a
ar: two different operation options specified
% ar -plugin=/usr/lib/gcc/x86_64-linux-gnu/10/liblto_plugin.so x x.a
ar: x: No such file or directory
```

Drop one-dash long options to simplify code and match the usual practice for
command line utilities.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D116977