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.

gas/ChangeLog

+2018-02-20  Thomas Preud'homme  <thomas.preudhomme@arm.com>
+
+	* doc/c-arm.texi (.arch_extension): Mention extensions it accepts are
+	also the same as -march.
+
 2018-02-17  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* NEWS: Mention .nop directive.

gas/doc/c-arm.texi

@@ -631,7 +631,7 @@ extensions.
 @item .arch_extension @var{name}
 Add or remove an architecture extension to the target architecture.  Valid
 values for @var{name} are the same as those accepted as architectural
-extensions by the @option{-mcpu} commandline option.
+extensions by the @option{-mcpu} and @option{-march} commandline options.
 
 @code{.arch_extension} may be used multiple times to add or remove extensions
 incrementally to the architecture being compiled for.