runtime/cgo: use PTHREAD_{MUTEX,COND}_INITIALIZER

Technically you must initialize static pthread_mutex_t and
pthread_cond_t variables with the appropriate INITIALIZER macro.  In
practice the default initializers are zero anyhow, but it's still good
code hygiene.

Change-Id: I517304b16c2c7943b3880855c1b47a9a506b4bdf
Reviewed-on: https://go-review.googlesource.com/9433Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/cgo/gcc_libinit.c

@@ -10,8 +10,8 @@
 #include <stdlib.h>
 #include <string.h> // strerror
 
-static pthread_cond_t runtime_init_cond;
-static pthread_mutex_t runtime_init_mu;
+static pthread_cond_t runtime_init_cond = PTHREAD_COND_INITIALIZER;
+static pthread_mutex_t runtime_init_mu = PTHREAD_MUTEX_INITIALIZER;
 static int runtime_init_done;
 
 void