iOS config targets: disable "async" by default

This also gave enough reason to collect the stuff that's common for
all iOS config targets into the template "ios-common".

Fixes #7318

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7403)

(cherry picked from commit 04e3f9a114c2c142356ef1639d68397a72e0c7f8)