[AArch64] Add feature flags and command line for ARMv8.2 FP16 support.

[Nick Clifton]

Hi Matthew,

> There are currently no FP16 instructions implemented in binutils, this
> patch is to enable subsequent work on supporting the extension.

If this subsequent work is going to take some time, then please could 
you update the patch to gas/doc/c-aarch.texi to point out that although 
the command line option is active the feature is not yet implemented.

> gas/
> 2015-11-24  Matthew Wahab  <matthew.wahab@arm.com>
>
>      * config/tc-aarch64.c (aarch64_features): Add "fp16".
>      * doc/c-aarch64.texi (Architecture Extensions): Add "fp16".
>
> include/opcode/
> 2015-11-24  Matthew Wahab  <matthew.wahab@arm.com>
>
>      * aarch64.h (AARCH64_FEATURE_F16): New.
>      (AARCH64_ARCH_V8_2): Add AARCH64_FEATURE_F16 to ARMv8.2
>      features.

Approved - please apply - with the caveat given above.

Cheers
   Nick