Use getcontextx(3) internal API instead of getcontext(2) to provide

the signal handlers with the context information in the deferrred
case.

Only enable the use of getcontextx(3) in the deferred signal delivery
code on amd64 and i386. Sparc64 seems to have some undetermined issues
with interaction of alloca(3) and signal delivery.

Tested by:	flo (who also provided sparc64 harware access for me), pho
Discussed with:	marius
MFC after:	1 month

lib/libthr/thread/thr_sig.c

@@ -32,6 +32,7 @@
 #include <sys/signalvar.h>
 #include <signal.h>
 #include <errno.h>
+#include <stdlib.h>
 #include <string.h>
 #include <pthread.h>
 #include "un-namespace.h"
@@ -314,16 +315,24 @@ check_cancel(struct pthread *curthread, ucontext_t *ucp)
 static void
 check_deferred_signal(struct pthread *curthread)
 {
-	ucontext_t uc;
+	ucontext_t *uc;
 	struct sigaction act;
 	siginfo_t info;
 
 	if (__predict_true(curthread->deferred_siginfo.si_signo == 0))
 		return;
-	getcontext(&uc);
+
+#if defined(__amd64__) || defined(__i386__)
+	uc = alloca(__getcontextx_size());
+	__fillcontextx((char *)uc);
+#else
+	ucontext_t ucv;
+	uc = &ucv;
+	getcontext(uc);
+#endif
 	if (curthread->deferred_siginfo.si_signo != 0) {
 		act = curthread->deferred_sigact;
-		uc.uc_sigmask = curthread->deferred_sigmask;
+		uc->uc_sigmask = curthread->deferred_sigmask;
 		memcpy(&info, &curthread->deferred_siginfo, sizeof(siginfo_t));
 		/* remove signal */
 		curthread->deferred_siginfo.si_signo = 0;
@@ -334,7 +343,7 @@ check_deferred_signal(struct pthread *curthread)
 			tact.sa_handler = SIG_DFL;
 			_sigaction(info.si_signo, &tact, NULL);
 		}
-		handle_signal(&act, info.si_signo, &info, &uc);
+		handle_signal(&act, info.si_signo, &info, uc);
 	}
 }