[i386, doc] Add documentation for fxsr, xsave, xsaveopt

[Uros Bizjak]

Hello!

> I've added references to fxsr, xsave and xsaveopt options and builtins
> to doc/[invoke,extend].texi.
>
> Is it OK?
>
> 2013-09-10  Alexander Ivchenko  <alexander.ivchenko@intel.com>
>
>  * doc/invoke.texi: Document fxsr, xsave and xsaveopt options.
>  * doc/extend.texi: Document fxsr, xsave and xsaveopt builtins.

Please put ChangeLog entry in the body of the mail message.

The patch is OK for mainline.

Thanks,
Uros.