BuildSystem: Move all userpace libraries under the userspace directory

As the number of libraries is increasing, root directory starts to
expand. This adds better organization for libraries

userspace/libraries/LibC/signal.cpp

@@ -0,0 +1,20 @@
+#include <signal.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+int raise(int sig)
+{
+	// FIXME: won't work after multithreaded
+	return kill(getpid(), sig);
+}
+
+int kill(pid_t pid, int sig)
+{
+	return syscall(SYS_KILL, pid, sig);
+}
+
+void (*signal(int sig, void (*func)(int)))(int)
+{
+	long ret = syscall(SYS_SIGNAL, sig, func);
+	return (void (*)(int))ret;
+}