arm: ignore inapplicable .arch=no...

Unlike for command line options, where a base architecture needs to be
provided explicitly, the .arch directive doesn't have such a
requirement. Therefore it is odd that disabling of an inapplicable
extension isn't silently ignored; claiming "not allowed for the current
base architecture" is at best misleading. Alter the error path to emit a
more "soft" diagnostic in that case instead.