Auto merge of #116448 - Kobzol:bootstrap-host-flags, r=onur-ozkan,petrochenkov

Pass rustc shim flags using environment variable

This PR implements a generalized way of passing of host flags to the `rustc` shim in bootstrap, as proposed [here](rust-lang/rust#116278 (comment)).

I tried to implement the bootstrap side using `OsString`, but then I realized that the shim code was using `env::var` before anyway, instead of `env::var_os`, so I just settled on a `String`. The shim side is still general and uses `env::vars_os` now.

I'm not sure if we actually need to do something with the `rustdoc` shim. It *seems* to me that the env. vars passed to it (`RUSTDOC_LINKER`) and (`RUSTDOC_LLD_NO_THREADS`) could just be passed to cargo directly (or rather, the commands that they invoke in the shim could be passed directly). I'm not sure why are they set by the shim.

r? `@onur-ozkan`
CC `@petrochenkov`

Author: bors