Define BORINGSSL_SHARED_LIBRARY when building tests with Bazel.

Bazel builds tests as shared libraries and the new p256-x86_64_test
depends on accessing unexported symbols. Thus we need to define
BORINGSSL_SHARED_LIBRARY when building tests.

Change-Id: I1270c69ac9d1bcf6baa05ef6666078bd368d80cf
Reviewed-on: https://boringssl-review.googlesource.com/12360
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>
diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 0b84bfd..aae158d 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -266,7 +266,7 @@
               out.write('          "%s",\n' % arg)
           out.write('      ],\n')
 
-        out.write('      copts = copts,\n')
+        out.write('      copts = copts + ["-DBORINGSSL_SHARED_LIBRARY"],\n')
 
         if len(data_files) > 0:
           out.write('      data = [\n')