[PAC][libunwind] Fix gcc build of libunwind and compiler-rt (llvm#164535)

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

Author: ojhunt

compiler-rt/lib/builtins/gcc_personality_v0.c

@@ -30,6 +30,10 @@ EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD, void *, PCONTEXT,
                                             _Unwind_Personality_Fn);
 #endif
 
+#ifndef __has_feature
+#define __has_feature(__feature) 0
+#endif
+
 #if __has_feature(ptrauth_calls)
 #include <ptrauth.h>
 

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 +634,10 @@ Lnovec:
 
 #elif defined(__aarch64__)
 
+#ifndef __has_feature
+#define __has_feature(__feature) 0
+#endif
+
 #if defined(__ARM_FEATURE_GCS_DEFAULT)
 .arch_extension gcs
 #endif

libunwind/src/UnwindRegistersSave.S

@@ -763,6 +763,10 @@ LnoR2Fix:
 
 #elif defined(__aarch64__)
 
+#ifndef __has_feature
+#define __has_feature(__feature) 0
+#endif
+
 //
 // extern int __unw_getcontext(unw_context_t* thread_state)
 //