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_session/src/config.rs

@@ -1319,6 +1319,7 @@ pub enum PAuthKey {
 #[derive(Clone, Copy, Hash, Debug, PartialEq)]
 pub struct PacRet {
     pub leaf: bool,
+    pub pc: bool,
     pub key: PAuthKey,
 }