Rollup merge of #135927 - azhogin:azhogin/retpoline, r=davidtwco

retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features

`-Zretpoline` and `-Zretpoline-external-thunk` flags are target modifiers (tracked to be equal in linked crates).
* Enables target features for `-Zretpoline-external-thunk`:
`+retpoline-external-thunk`, `+retpoline-indirect-branches`, `+retpoline-indirect-calls`.
* Enables target features for `-Zretpoline`:
`+retpoline-indirect-branches`, `+retpoline-indirect-calls`.

It corresponds to clang -mretpoline & -mretpoline-external-thunk flags.

Also this PR forbids to specify those target features manually (warning).

Issue: rust-lang/rust#116852

Author: matthiaskrgr