sigchain: filter out sigprocmask(SIG_SETMASK, ...).

Previously, we were only filtering out signals blocked via SIG_BLOCK...

Test: adb shell /data/nativetest64/art/arm64/art_sigchain_tests/sigchain_test
Change-Id: Iee2284442f744431a4216ab15aa0ef2fd4d58599
diff --git a/sigchainlib/sigchain.cc b/sigchainlib/sigchain.cc
index 85482e3..6d3029d 100644
--- a/sigchainlib/sigchain.cc
+++ b/sigchainlib/sigchain.cc
@@ -446,7 +446,7 @@
   if (new_set != nullptr) {
     tmpset = *new_set;
 
-    if (how == SIG_BLOCK) {
+    if (how == SIG_BLOCK || how == SIG_SETMASK) {
       // Don't allow claimed signals in the mask.  If a signal chain has been claimed
       // we can't allow the user to block that signal.
       for (int i = 0 ; i < _NSIG; ++i) {