Cronet 121.0.6167.71: set hardening mode for boringssl

libc++ started requiring hardening mode for our c++ targets.
llvm/llvm-project#70575
This only started failing since they removed the default
hardening mode.

Chromium switched to HARDENING_MODE_EXTENSIVE in https://crrev.com/c/5014271

Bug: b/322154153
Change-Id: I538654e05ddbee3120e13f7b19df72097b77bb6d

Author: colibie

third_party/boringssl/Android.bp

@@ -64,6 +64,8 @@ cc_defaults {
         "-fvisibility=hidden",
         "-DBORINGSSL_SHARED_LIBRARY",
         "-DBORINGSSL_ANDROID_SYSTEM",
+        // Chromium uses extensive harderning mode, so setting the same for boringssl.
+        "-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE",
         "-DOPENSSL_SMALL",
         "-Werror",
         "-Wno-unused-parameter",