Retpoline support in `rustc` (target features or dedicated flags)

[ojeda]

Currently:

-Ctarget-feature="+retpoline-external-thunk"
-Ctarget-feature="+retpoline-indirect-branches"
-Ctarget-feature="+retpoline-indirect-calls"

seems to work as expected, but the features are unknown, and so since 1.61 they emit a warning:

warning: unknown feature specified for `-Ctarget-feature`: `retpoline-external-thunk`
  |
  = note: it is still passed through to the codegen backend
  = help: consider filing a feature request

warning: unknown feature specified for `-Ctarget-feature`: `retpoline-indirect-branches`
  |
  = note: it is still passed through to the codegen backend
  = help: consider filing a feature request

warning: unknown feature specified for `-Ctarget-feature`: `retpoline-indirect-calls`
  |
  = note: it is still passed through to the codegen backend
  = help: consider filing a feature request

Could/should these be added as known features to avoid using the target specification file?

In an old issue at #54637 it seems that back then dedicated flags were not intended (like Clang's -mretpoline-external-thunk and GCC's -mindirect-branch=thunk-extern -mindirect-branch-register), which perhaps make a bit more sense.

PR: #135927 (1.89).
PR: #140740 (1.91).
MCP: rust-lang/compiler-team#868 (-Zretpoline and -Zretpoline-external-thunk).
MCP: rust-lang/compiler-team#899 (-Zindirect-branch-cs-prefix).