Guard GCC-specific macros with !defined(__clang__)

By default Clang on Linux defines the macro `__GNUC__`, so to guard GCC-specific
code paths it isn't sufficient to check `#ifdef __GNUC__`.

src/julia_internal.h

@@ -422,7 +422,8 @@ jl_svec_t *jl_perm_symsvec(size_t n, ...);
 
 // this sizeof(__VA_ARGS__) trick can't be computed until C11, but that only matters to Clang in some situations
 #if !defined(__clang_analyzer__) && !(defined(_COMPILER_ASAN_ENABLED_) || defined(_COMPILER_TSAN_ENABLED_))
-#ifdef __GNUC__
+// Following macros are only for GCC, but Clang on Linux may define `__GNUC__`, so we have to exclude it.
+#if defined(__GNUC__) && !defined(__clang__)
 #define jl_perm_symsvec(n, ...) \
     (jl_perm_symsvec)(__extension__({                                         \
             static_assert(                                                    \