Clarify .arch_extension possible values

Clarify .arch_extension possible values

Documentation for .arch_extension says it accepts the same architectural
extensions as those accepted by -mcpu. Given the name and the fact that
-march for obvious reason also accept the same extensions, I believe
it's worth mentioning that it accepts the same extensions as both
-march and -mcpu. This commit addresses that.

2018-02-20  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* doc/c-arm.texi (.arch_extension): Mention extensions it accepts are
	also the same as -march.