runtime: unblock signals when we try to core dump
Fixes #6988.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/44070046
diff --git a/src/pkg/runtime/os_netbsd.c b/src/pkg/runtime/os_netbsd.c
index a49dca2..7458181 100644
--- a/src/pkg/runtime/os_netbsd.c
+++ b/src/pkg/runtime/os_netbsd.c
@@ -326,3 +326,9 @@
 		st.ss_flags = SS_DISABLE;
 	runtime·sigaltstack(&st, nil);
 }
+
+void
+runtime·unblocksignals(void)
+{
+	runtime·sigprocmask(SIG_SETMASK, &sigset_none, nil);
+}