reactor: access sigevent.sigev_notify_thread_id with a macro

sigev_notify_thread_id is defined as a macro by musl libc, FreeBSD, and the Linux kernel.
The macro is for accessing the corresponding field in sigevent.

see https://sourceware.org/bugzilla/show_bug.cgi?id=27417

src/core/reactor.cc

@@ -1176,7 +1176,10 @@ cpu_stall_detector_posix_timer::cpu_stall_detector_posix_timer(cpu_stall_detecto
     struct sigevent sev = {};
     sev.sigev_notify = SIGEV_THREAD_ID;
     sev.sigev_signo = signal_number();
-    sev._sigev_un._tid = syscall(SYS_gettid);
+#ifndef sigev_notify_thread_id
+#define sigev_notify_thread_id _sigev_un._tid
+#endif
+    sev.sigev_notify_thread_id = syscall(SYS_gettid);
     int err = timer_create(CLOCK_THREAD_CPUTIME_ID, &sev, &_timer);
     if (err) {
         throw std::system_error(std::error_code(err, std::system_category()));