Change `SYS_gettid` to __NR_gettid instead

Summary: SYS_gettid is not available on all platforms, but __NR_gettid should be.

Reviewed By: dcolascione

Differential Revision: D2787003

fb-gh-sync-id: ee024437ac95281a3573e2440653847f6f7d1738

folly/experimental/fibers/Fiber.cpp

@@ -38,7 +38,7 @@ pid_t localThreadId() {
   // OSX doesn't support thread_local.
   static FOLLY_TLS pid_t threadId = 0;
   if (UNLIKELY(threadId == 0)) {
-    threadId = syscall(SYS_gettid);
+    threadId = syscall(__NR_gettid);
   }
   return threadId;
 }