Don't catch fork()ing events for Native Client.

Native Client doesn't allow forking, thus there is no need to catch
fork()ing events for Native Client.

Additionally, without this commit, jemalloc will introduce an unresolved
pthread_atfork() in PNaCl Rust bins.

src/jemalloc.c

@@ -792,7 +792,7 @@ malloc_init_hard(void)
 	ncpus = malloc_ncpus();
 
 #if (!defined(JEMALLOC_MUTEX_INIT_CB) && !defined(JEMALLOC_ZONE) \
-    && !defined(_WIN32))
+    && !defined(_WIN32) && !defined(__native_client__))
 	/* LinuxThreads's pthread_atfork() allocates. */
 	if (pthread_atfork(jemalloc_prefork, jemalloc_postfork_parent,
 	    jemalloc_postfork_child) != 0) {