[ruby/json] Tentative fix for RHEL8 compiler

```
parser.c:87:77: error: missing binary operator before token "("
 #if JSON_CPU_LITTLE_ENDIAN_64BITS && defined(__has_builtin) && __has_builtin(__builtin_bswap64)
```

ruby/json@fce1c7e84a

Author: byroot

ext/json/parser/parser.c

@@ -84,7 +84,11 @@ static void rvalue_cache_insert_at(rvalue_cache *cache, int index, VALUE rstring
     cache->entries[index] = rstring;
 }
 
-#if JSON_CPU_LITTLE_ENDIAN_64BITS && defined(__has_builtin) && __has_builtin(__builtin_bswap64)
+#define rstring_cache_memcmp memcmp
+
+#if JSON_CPU_LITTLE_ENDIAN_64BITS
+#if __has_builtin(__builtin_bswap64)
+#undef rstring_cache_memcmp
 static ALWAYS_INLINE() int rstring_cache_memcmp(const char *str, const char *rptr, const long length)
 {
     // The libc memcmp has numerous complex optimizations, but in this particular case,
@@ -111,8 +115,7 @@ static ALWAYS_INLINE() int rstring_cache_memcmp(const char *str, const char *rpt
 
     return 0;
 }
-#else
-#define rstring_cache_memcmp memcmp
+#endif
 #endif
 
 static ALWAYS_INLINE() int rstring_cache_cmp(const char *str, const long length, VALUE rstring)

ext/json/simd/simd.h

@@ -6,6 +6,10 @@ typedef enum {
     SIMD_SSE2
 } SIMD_Implementation;
 
+#ifndef __has_builtin         // Optional of course.
+  #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
+#endif
+
 #ifdef __clang__
 # if __has_builtin(__builtin_ctzll)
 #   define HAVE_BUILTIN_CTZLL 1