-Zharden-sls flag (target modifier) added to enable mitigation against straight line speculation (SLS)

[azhogin]

Flag (target modifier) to mitigate against straight line speculation (SLS).
-Zharden-sls=[none|all|return|indirect-jmp].

The flag enables the related features: +harden-sls-ijmp,+harden-sls-ret.
The flag is tracked as a target modifier to be equal between linked crates.

[rustbot]

r? @estebank

rustbot has assigned @estebank.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[klensy]

First commit adds retpoline feature, but PR descriptions says about sls.

[azhogin]

First commit adds retpoline feature, but PR descriptions says about sls.

Yes, first commit is in another PR #135927.
This one depends on.

[Dylan-DPC]

Marking this as blocked on #135927

[Darksonn]

I filed an MCP for this flag: rust-lang/compiler-team#869

[bors]

☔ The latest upstream changes (presumably #141232) made this pull request unmergeable. Please resolve the merge conflicts.