[Bug target/71235] march=silvermont turns on aes by default

[m.vorobyov at cs dot msu.ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71235

--- Comment #3 from Mikhail Vorobyov <m.vorobyov at cs dot msu.ru> ---
Maybe it should be said more precisely that -march isn't for all cores of
family but for only ones which have those extensions?