rustc_codegen_llvm: Add a new 'pc' option to branch-protection

Add a new 'pc' option to -Z branch-protection for aarch64 that
enables the use of PC as a diversifier in PAC branch protection code.

When the pauth-lr target feature is enabled in combination
with -Z branch-protection=pac-ret,pc, the new 9.5-a instructions
(pacibsppc, retaasppc, etc) will be generated.

compiler/rustc_interface/src/tests.rs

@@ -764,7 +764,7 @@ fn test_unstable_options_tracking_hash() {
         branch_protection,
         Some(BranchProtection {
             bti: true,
-            pac_ret: Some(PacRet { leaf: true, key: PAuthKey::B })
+            pac_ret: Some(PacRet { leaf: true, pc: true, key: PAuthKey::B })
         })
     );
     tracked!(codegen_backend, Some("abc".to_string()));