[Patch, AArch64] Add GAS target-specific command line option -mabi=<abi_name>

[nick clifton]

Hi Yufeng,

>>     * You should issue an error message if the old (now deprecated)
>> -milp32 or -mlp64 command line options are used.
>
> I agree that in general the deprecation shall communicate more friendly
> error message to the existing usage, but in this particular case, I
> think it is less required to do so as it is very unlikely -milp32 and
> -mlp64 have been used by the community so far.  The options were added
> only a few weeks ago and the related patch for the gcc driver has not
> been committed to the upstream yet.  So I think it shall be fine to
> simply remove the support for the two options.  What do you think?
>

Oh fine - I did not check how long the options had been present.  What 
you say makes sense, so please ignore my comment on this part of the patch.

Cheers
   Nick