[RFA/ARM] Add support for // comments

[nick clifton]

Hi Matt,

> gas/ChangeLog:
>
> 2012-02-09  Matthew Gretton-Dann<matthew.gretton-dann@arm.com>
>
> 	* gas/config/tc-arm.h (DOUBLESLASH_LINE_COMMENTS): Define.

Approved and applied.

Cheers
   Nick