external/boringssl: Sync to 8ebeabf0e2e01b331e56d0a491c12539baa55d3d.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/be2ee342d3781ddb954f91f8a7e660c6f59e87e5..8ebeabf0e2e01b331e56d0a491c12539baa55d3d

Test: Libcore CTS presubmits.
Change-Id: I2fefc3e2bc2bbc3e3083668bd2a56d491520bc24
diff --git a/src/crypto/dh/check.c b/src/crypto/dh/check.c
index f40e034..e3c111b 100644
--- a/src/crypto/dh/check.c
+++ b/src/crypto/dh/check.c
@@ -93,7 +93,7 @@
     /* Check |pub_key|^|dh->q| is 1 mod |dh->p|. This is necessary for RFC 5114
      * groups which are not safe primes but pick a generator on a prime-order
      * subgroup of size |dh->q|. */
-    if (!BN_mod_exp(tmp, pub_key, dh->q, dh->p, ctx)) {
+    if (!BN_mod_exp_mont(tmp, pub_key, dh->q, dh->p, ctx, NULL)) {
       goto err;
     }
     if (!BN_is_one(tmp)) {
@@ -145,7 +145,7 @@
       *ret |= DH_CHECK_NOT_SUITABLE_GENERATOR;
     } else {
       /* Check g^q == 1 mod p */
-      if (!BN_mod_exp(t1, dh->g, dh->q, dh->p, ctx)) {
+      if (!BN_mod_exp_mont(t1, dh->g, dh->q, dh->p, ctx, NULL)) {
         goto err;
       }
       if (!BN_is_one(t1)) {