Auto merge of rust-lang#149148 - davidtwco:v0-mangling-on-nightly-std, r=Kobzol

v0 mangling for std on nightly

Following rust-lang#89917 and rust-lang/compiler-team#938, it doesn't make sense that `std` for these channels would have legacy mangling while the user's code would use `v0`.

r? `@Kobzol`

Author: bors