add getrandom syscall wrapper

This syscall is available since Linux 3.17 and was also implemented in
glibc in version 2.25 using the same interfaces.

src/linux/getrandom.c

@@ -0,0 +1,7 @@
+#include <sys/random.h>
+#include "syscall.h"
+
+ssize_t getrandom(void *buf, size_t buflen, unsigned flags)
+{
+	return syscall_cp(SYS_getrandom, buf, buflen, flags);
+}