Define SIGUSR1 for hybrid Linux/Genode programs

This patch fixes the 'lx_hybrid_pthread_ipc.run' test. In order to use
the 'Genode::Lock' we need to set the SIGUSR1 handler to an empty handler.
Normally, this happens when creating a thread via the Genode API. But as
this test creates a thread via the pthread library and thereby bypasses
the Genode API, the signal handler remained unset.

base-linux/src/platform/lx_hybrid.cc

@@ -40,6 +40,8 @@ int genode___cxa_atexit(void (*func)(void*), void *arg, void *dso)
 extern char **environ;
 extern char **lx_environ;
 
+static void empty_signal_handler(int) { }
+
 /*
  * This function must be called before any other static constructor in the Genode
  * application, so it gets the highest priority (lowest priority number >100)
@@ -47,6 +49,12 @@ extern char **lx_environ;
 __attribute__((constructor(101))) void lx_hybrid_init()
 {
 	lx_environ = environ;
+
+	/*
+	 * Set signal handler such that canceled system calls get not
+	 * transparently retried after a signal gets received.
+	 */
+	lx_sigaction(LX_SIGUSR1, empty_signal_handler);
 }
 
 /*
@@ -158,9 +166,6 @@ namespace Genode {
 }
 
 
-static void empty_signal_handler(int) { }
-
-
 /**
  * Return Linux-specific extension of the Env::CPU session interface
  */