[PATCH, binutils/ARM] Clarify .arch_extension possible values

[Nick Clifton]

Hi Thomas,

> 2018-02-19  Thomas Preud'homme  <thomas.preudhomme@arm.com>
> 
> Â Â Â Â * doc/c-arm.texi (.arch_extension): Mention extensions it accepts are
> Â Â Â Â also the same as -march.

Approved - please apply.

Cheers
  Nick