runtime/cgo: always set signal mask before calling pthread_create

This was done correctly for most targets but was missing from
FreeBSD/ARM and Linux/ARM.

R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/45180043

src/pkg/runtime/cgo/gcc_freebsd_arm.c

@@ -39,10 +39,14 @@ void
 _cgo_sys_thread_start(ThreadStart *ts)
 {
 	pthread_attr_t attr;
+	sigset_t ign, oset;
 	pthread_t p;
 	size_t size;
 	int err;
 
+	SIGFILLSET(ign);
+	pthread_sigmask(SIG_SETMASK, &ign, &oset);
+
 	// Not sure why the memset is necessary here,
 	// but without it, we get a bogus stack size
 	// out of pthread_attr_getstacksize.  C'est la Linux.
@@ -52,6 +56,9 @@ _cgo_sys_thread_start(ThreadStart *ts)
 	pthread_attr_getstacksize(&attr, &size);
 	ts->g->stackguard = size;
 	err = pthread_create(&p, &attr, threadentry, ts);
+
+	pthread_sigmask(SIG_SETMASK, &oset, nil);
+
 	if (err != 0) {
 		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
 		abort();

src/pkg/runtime/cgo/gcc_linux_arm.c

@@ -28,10 +28,14 @@ void
 _cgo_sys_thread_start(ThreadStart *ts)
 {
 	pthread_attr_t attr;
+	sigset_t ign, oset;
 	pthread_t p;
 	size_t size;
 	int err;
 
+	sigfillset(&ign);
+	pthread_sigmask(SIG_SETMASK, &ign, &oset);
+
 	// Not sure why the memset is necessary here,
 	// but without it, we get a bogus stack size
 	// out of pthread_attr_getstacksize.  C'est la Linux.
@@ -41,6 +45,9 @@ _cgo_sys_thread_start(ThreadStart *ts)
 	pthread_attr_getstacksize(&attr, &size);
 	ts->g->stackguard = size;
 	err = pthread_create(&p, &attr, threadentry, ts);
+
+	pthread_sigmask(SIG_SETMASK, &oset, nil);
+
 	if (err != 0) {
 		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
 		abort();