Enable back-edge CFI by default on macOS (#4720)

Also, adjust the tests that are executed on that platform. Finally,
fix a bug with obtaining backtraces when back-edge CFI is enabled.

Copyright (c) 2022, Arm Limited.

cranelift/codegen/src/isa/aarch64/inst.isle

@@ -780,6 +780,11 @@
        (Pacisp
         (key APIKey))
 
+       ;; Strip pointer authentication code from instruction address in LR;
+       ;; equivalent to a no-op if Pointer authentication (FEAT_PAuth) is not
+       ;; supported.
+       (Xpaclri)
+
        ;; Marker, no-op in generated code: SP "virtual offset" is adjusted. This
        ;; controls how AMode::NominalSPOffset args are lowered.
        (VirtualSPOffsetAdj
@@ -1356,6 +1361,9 @@
 ))
 
 ;; Extractors for target features ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+(decl pure sign_return_address_disabled () Unit)
+(extern constructor sign_return_address_disabled sign_return_address_disabled)
+
 (decl use_lse () Inst)
 (extern extractor use_lse use_lse)
 
@@ -2577,4 +2585,11 @@
 
 (decl aarch64_link () Reg)
 (rule (aarch64_link)
+      (if (sign_return_address_disabled))
       (mov_preg (preg_link)))
+
+(rule (aarch64_link)
+      ;; This constructor is always used for non-leaf functions, so it is safe
+      ;; to clobber LR.
+      (let ((_ Unit (emit (MInst.Xpaclri))))
+           (mov_preg (preg_link))))