Since gcm_test now contains variable decls in for loops it needs
-std=c11. However, tests are compiled with C++ test_support files in
Bazel, which doesn't work with -std=c11.

Change-Id: Ife18c2d80b01448bb3b7ee2728412289bf749bd9
Reviewed-on: https://boringssl-review.googlesource.com/11041
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>