Clarify doc for .arch/.cpu

Clarify doc for .arch/.cpu

Documentation for .arch and .cpu directives currently says that it
accepts the same name as -march/-mcpu command-line options respectively.
However it only accept the architecture/CPU part of those options: it
does not accept specifying an extension which is done via
.arch_extension. This patch clarifies that the extension is not
accepted.

2018-07-25  Thomas Preud'homme  <thomas.preudhomme@linaro.org>

gas/
	* doc/c-arm.texi (.arch directive): Clarify that name must not include
	an extension.
	(.cpu directive): Likewise.