linux.syscall_pwait failing on Android.

Test output was

handlers for signals: 10, 12 signal blocked: 10 Testing epoll_pwait signal received: 12 signal received: 10 Testing pselect signal received: 12 Testing ppoll signal received: 12 Testing epoll_pwait failure Testing pselect failure Testing ppoll failure

Seems that the SIGUSR1 signals were not received.