[clang] [RISCV] Implement shadow stack on shadow stack mode with Zicfiss. (PR #68075)

[Paul Kirth via cfe-commits]

================
@@ -106,9 +111,14 @@ static void emitSCSEpilogue(MachineFunction &MF, MachineBasicBlock &MBB,
           CSI, [&](CalleeSavedInfo &CSR) { return CSR.getReg() == RAReg; }))
     return;
 
+  const RISCVInstrInfo *TII = STI.getInstrInfo();
+  if (STI.hasFeature(RISCV::FeatureStdExtZicfiss)) {
----------------
ilovepi wrote:

I think this is probably handled by the loader. Its how it is currently handled for platforms like Fuchsia and Android when SCS is enabled. I'm not totally sure on the new spec, but the linked document above makes me think that is the case with this text:

```
The dynamic loader should activate the use of Zicfiss extension for an application only if all executables (the application and the dependent dynamically linked libraries) used by that application use the Zicfiss extension.

An application that has the Zicfiss extension active may request the dynamic loader at runtime to load a new dynamic shared object (using dlopen() for example). If the requested object does not have the Zicfiss attribute then the dynamic loader, based on its policy (e.g, established by the operating system or the administrator) configuration, could either deny the request or deactivate the Zicfiss extension for the application. It is strongly recommended that the policy enforces a strict security posture and denies the request.
```

I believe that setting up this memory and initializing the SCS reg would fall under `activate the use of Zicfiss...`, but I could be wrong.

https://github.com/llvm/llvm-project/pull/68075