[PAC][libunwind] Fix gcc build of libunwind

This adds guards on the ptrauth feature checks so that they are
only performed if __has_feature is actually available.

Author: ojhunt

libunwind/include/__libunwind_config.h

@@ -212,11 +212,13 @@
 # define _LIBUNWIND_HIGHEST_DWARF_REGISTER 287
 #endif // _LIBUNWIND_IS_NATIVE_ONLY
 
-#if __has_feature(ptrauth_calls) && __has_feature(ptrauth_returns)
-#  define _LIBUNWIND_TARGET_AARCH64_AUTHENTICATED_UNWINDING 1
-#elif __has_feature(ptrauth_calls) != __has_feature(ptrauth_returns)
-#  error "Either both or none of ptrauth_calls and ptrauth_returns "\
-         "is allowed to be enabled"
+#if defined(__has_feature)
+#  if __has_feature(ptrauth_calls) && __has_feature(ptrauth_returns)
+#    define _LIBUNWIND_TARGET_AARCH64_AUTHENTICATED_UNWINDING 1
+#  elif __has_feature(ptrauth_calls) != __has_feature(ptrauth_returns)
+#    error "Either both or none of ptrauth_calls and ptrauth_returns "\
+           "is allowed to be enabled"
+#  endif
 #endif
 
 #endif // ____LIBUNWIND_CONFIG_H__

libunwind/src/UnwindRegistersRestore.S

@@ -634,6 +634,12 @@ Lnovec:
 
 #elif defined(__aarch64__)
 
+#ifdef __has_feature
+#define __has_ptrauth_calls __has_feature(ptrauth_calls)
+#else
+#define __has_ptrauth_calls 0
+#endif
+
 #if defined(__ARM_FEATURE_GCS_DEFAULT)
 .arch_extension gcs
 #endif
@@ -690,7 +696,7 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
   ldr    x16,     [x0, #0x0F8]  // load sp into scratch
   ldr    lr,      [x0, #0x100]  // restore pc into lr
 
-#if __has_feature(ptrauth_calls)
+#if __has_ptrauth_calls
   // The LR is signed with its address inside the register state.  Time
   // to resign to be a regular ROP protected signed pointer
   add    x1, x0, #0x100
@@ -711,7 +717,7 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
 Lnogcs:
 #endif
 
-#if __has_feature(ptrauth_calls)
+#if __has_ptrauth_calls
   retab
 #else
   ret    x30                    // jump to pc

libunwind/src/UnwindRegistersSave.S

@@ -763,6 +763,12 @@ LnoR2Fix:
 
 #elif defined(__aarch64__)
 
+#ifdef __has_feature
+#define __has_ptrauth_calls __has_feature(ptrauth_calls)
+#else
+#define __has_ptrauth_calls 0
+#endif
+
 //
 // extern int __unw_getcontext(unw_context_t* thread_state)
 //
@@ -772,7 +778,7 @@ LnoR2Fix:
   .p2align 2
 DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
 
-#if __has_feature(ptrauth_calls)
+#if __has_ptrauth_calls
   pacibsp
 #endif
 
@@ -817,7 +823,7 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
 #endif
   mov    x0, #0                   // return UNW_ESUCCESS
 
-#if __has_feature(ptrauth_calls)
+#if __has_ptrauth_calls
   retab
 #else
   ret