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

[nick clifton]

Hi Yufeng,

> This patch replaces the gas command line options -milp32 and -mlp64 with
> -mabi=ilp32 and -mabi=lp64 respectively.  This will unify the ABI
> command line user interface with that of the compiler.  Separate patch
> updating the compiler driver will be sent shortly.

I see a couple of small problems with this patch:

   * You should issue an error message if the old (now deprecated) 
-milp32 or -mlp64 command line options are used.


   * You are testing membership of an enum using a binary AND operator:

#define ilp32_p		(aarch64_abi & AARCH64_ABI_ILP32)

   Ie, I think that this should be:

#define ilp32_p		(aarch64_abi == AARCH64_ABI_ILP32)


Other than that, the patch looks fine.

Cheers
   Nick