Revert mutex change.

node/Mutex.hpp

@@ -37,8 +37,7 @@
 
 namespace ZeroTier {
 
-//#if defined(__GNUC__) && (defined(__amd64) || defined(__amd64__) || defined(__x86_64) || defined(__x86_64__) || defined(__AMD64) || defined(__AMD64__) || defined(_M_X64))
-#if 0
+#if defined(__GNUC__) && (defined(__amd64) || defined(__amd64__) || defined(__x86_64) || defined(__x86_64__) || defined(__AMD64) || defined(__AMD64__) || defined(_M_X64))
 
 // Inline ticket lock on x64 systems with GCC and CLANG (Mac, Linux) -- this is really fast as long as locking durations are very short
 class Mutex