make RNG state thread_local

src/common/broadwell/randombytes_ctrdrbg_aesni.c

@@ -33,6 +33,7 @@ protection within the United States.
 #include <string.h>
 
 #include <rng.h>
+#include <threads.h>
 #include "ctr_drbg.h"
 
 #ifdef ENABLE_CT_TESTING
@@ -44,7 +45,7 @@ protection within the United States.
 #define RNG_BAD_OUTBUF -2
 #define RNG_BAD_REQ_LEN -3
 
-CTR_DRBG_STATE drbg;
+thread_local CTR_DRBG_STATE drbg;
 
 #ifndef CTRDRBG_TEST_BENCH
 static