fast_float_parser.h: fix GCC 8 support

Author: byroot

ext/json/ext/vendor/fast_float_parser.h

@@ -720,7 +720,10 @@ static inline double ffp_bits2double(uint64_t bits) {
 static inline unsigned int
 ffc_nlz_int64(uint64_t x)
 {
-#if defined(_MSC_VER) && defined(__AVX2__) && defined(HAVE___LZCNT64)
+#if defined(HAVE_BUILTIN___BUILTIN_CLZLL) && !(defined(__sun) && defined(__sparc))
+    return (unsigned int)__builtin_clzll((unsigned long long)x);
+
+#elif defined(_MSC_VER) && defined(__AVX2__) && defined(HAVE___LZCNT64)
     return (unsigned int)__lzcnt64(x);
 
 #elif defined(__x86_64__) && defined(__LZCNT__) && defined(HAVE__LZCNT_U64)
@@ -730,21 +733,6 @@ ffc_nlz_int64(uint64_t x)
     unsigned long r;
     return _BitScanReverse64(&r, x) ? (63u - (unsigned int)r) : 64;
 
-#elif defined(__has_builtin) && __has_builtin(__builtin_clzl) && __has_builtin(__builtin_clzll) && !(defined(__sun) && defined(__sparc))
-    if (x == 0) {
-        return 64;
-    }
-    else if (sizeof(long) * CHAR_BIT == 64) {
-        return (unsigned int)__builtin_clzl((unsigned long)x);
-    }
-    else if (sizeof(long long) * CHAR_BIT == 64) {
-        return (unsigned int)__builtin_clzll((unsigned long long)x);
-    }
-    else {
-        /* :FIXME: Is there a way to make this branch a compile-time error? */
-        __builtin_unreachable();
-    }
-
 #else
     uint64_t y;
     unsigned int n = 64;