[llvm] [SystemZ][z/OS] Implement llvm.returnaddress for XPLINK (PR #89440)

[Ulrich Weigand via llvm-commits]

================
@@ -97,6 +106,11 @@ class SystemZELFFrameLowering : public SystemZFrameLowering {
     return usePackedStack(MF) ? SystemZMC::ELFCallFrameSize - 8 : 0;
   }
 
+  // Return the offset of the return address.
+  virtual int getReturnAddressOffset(MachineFunction &MF) const override {
----------------
uweigand wrote:

We don't normally use `virtual` on the function definitions, only on the declarations.

https://github.com/llvm/llvm-project/pull/89440