Auto merge of #7353 - alexcrichton:libstd-no-dylib, r=ehuss

Auto merge of #7353 - alexcrichton:libstd-no-dylib, r=ehuss

Don't build libstd as a `dylib`

This commit forcibly prevents Cargo from building the `std` crate as a
`dylib`, even though libstd upstream lists a `dylib` crate type. We
ideally want a first-class feature for doing this one day, but for now
we can just hack around with the manifests to ensure that the `dylib`
crate type never shows up. Note that this is only supported for libstd,
and it's also all part of the unstable details of building std.

Closes rust-lang/wg-cargo-std-aware#35