acle/hints: most hints require 'v6' rather than 'v6k'
acle/hints: most hints require 'v6' rather than 'v6k'

addresses https://github.com/rust-lang-nursery/stdsimd/pull/557#discussion_r255251241

rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch