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

[ojeda]

Currently:

-Ctarget-feature="+harden-sls-ijmp"
-Ctarget-feature="+harden-sls-ret"

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`: `harden-sls-ijmp`
  |
  = note: it is still passed through to the codegen backend
  = help: consider filing a feature request

warning: unknown feature specified for `-Ctarget-feature`: `harden-sls-ret`
  |
  = 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? Or perhaps as dedicated flags like GCC & Clang's -mharden-sls=all?

MCP: rust-lang/compiler-team#869