Make std::env::{Args, ArgsOs} Send and Sync

[SteveLauC]

#48004 requests to make these 2 types !Send and !Sync and #48005 implements it.

From this comment, at that time, they were really not Send and Sync, but now, it seems both Unix and Wndows are defining Args as:

struct Args {
     args: vec::IntoIter<OsString>
}

which has no reason to be !Send and !Sync, I am not sure about that cloudabi platform though given that I didn't find it in the source code.

If the Std allocates for these types on all the platforms(i.e., basically a Vec<OsString>), we can probably make them Send and Sync

[dev-ardi]

Why were they made !Send and !Sync in the first place? I haven't been able to find information about it.