add support for SIGEV_THREAD_ID timers

This is like SIGEV_SIGNAL, but targeted to a particular thread's
tid, rather than the process.

include/signal.h

@@ -180,14 +180,24 @@ struct sigevent {
 	union sigval sigev_value;
 	int sigev_signo;
 	int sigev_notify;
-	void (*sigev_notify_function)(union sigval);
-	pthread_attr_t *sigev_notify_attributes;
-	char __pad[56-3*sizeof(long)];
+	union {
+		char __pad[64 - 2*sizeof(int) - sizeof(union sigval)];
+		pid_t sigev_notify_thread_id;
+		struct {
+			void (*sigev_notify_function)(union sigval);
+			pthread_attr_t *sigev_notify_attributes;
+		} __sev_thread;
+	} __sev_fields;
 };
 
+#define sigev_notify_thread_id __sev_fields.sigev_notify_thread_id
+#define sigev_notify_function __sev_fields.__sev_thread.sigev_notify_function
+#define sigev_notify_attributes __sev_fields.__sev_thread.sigev_notify_attributes
+
 #define SIGEV_SIGNAL 0
 #define SIGEV_NONE 1
 #define SIGEV_THREAD 2
+#define SIGEV_THREAD_ID 4
 
 int __libc_current_sigrtmin(void);
 int __libc_current_sigrtmax(void);

src/time/timer_create.c

@@ -71,11 +71,15 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 	switch (evp ? evp->sigev_notify : SIGEV_SIGNAL) {
 	case SIGEV_NONE:
 	case SIGEV_SIGNAL:
+	case SIGEV_THREAD_ID:
 		if (evp) {
 			ksev.sigev_value = evp->sigev_value;
 			ksev.sigev_signo = evp->sigev_signo;
 			ksev.sigev_notify = evp->sigev_notify;
-			ksev.sigev_tid = 0;
+			if (evp->sigev_notify == SIGEV_THREAD_ID)
+				ksev.sigev_tid = evp->sigev_notify_thread_id;
+			else
+				ksev.sigev_tid = 0;
 			ksevp = &ksev;
 		}
 		if (syscall(SYS_timer_create, clk, ksevp, &timerid) < 0)
@@ -107,7 +111,7 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 
 		ksev.sigev_value.sival_ptr = 0;
 		ksev.sigev_signo = SIGTIMER;
-		ksev.sigev_notify = 4; /* SIGEV_THREAD_ID */
+		ksev.sigev_notify = SIGEV_THREAD_ID;
 		ksev.sigev_tid = td->tid;
 		if (syscall(SYS_timer_create, clk, &ksev, &timerid) < 0)
 			timerid = -1;