Adds AArch64 GCS support

- Adds option to rustc config to enable GCS
- Passes `guarded-control-stack` flag to llvm if enabled

compiler/rustc_codegen_llvm/src/context.rs

@@ -370,7 +370,8 @@ pub(crate) unsafe fn create_module<'ll>(
         );
     }
 
-    if let Some(BranchProtection { bti, pac_ret }) = sess.opts.unstable_opts.branch_protection {
+    if let Some(BranchProtection { bti, pac_ret, gcs }) = sess.opts.unstable_opts.branch_protection
+    {
         if sess.target.arch == "aarch64" {
             llvm::add_module_flag_u32(
                 llmod,
@@ -403,6 +404,12 @@ pub(crate) unsafe fn create_module<'ll>(
                 "sign-return-address-with-bkey",
                 u32::from(pac_opts.key == PAuthKey::B),
             );
+            llvm::add_module_flag_u32(
+                llmod,
+                llvm::ModuleFlagMergeBehavior::Min,
+                "guarded-control-stack",
+                gcs.into(),
+            );
         } else {
             bug!(
                 "branch-protection used on non-AArch64 target; \