Enable symbol prefixing for the Bazel build

It seems folks run into symbol conflicts between OpenSSL and BoringSSL
in Bazel packages:
https://github.com/bazelbuild/bazel/issues/26827

Given we have tools for this now, let's turn this on.

Just use a hardcoded "bssl_bzl" prefix because BCR doesn't let multiple
copies of a package get linked at once, so we don't have to incorporate
a version or anything complex like this.

Change-Id: I2bdce7370418240321c39267750e27af20391b40
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/98967
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Rudolf Polzer <rpolzer@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

BUILD.bazel

diff --git a/BUILD.bazel b/BUILD.bazel
index aef0732..ccfc818 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -27,8 +27,8 @@
     "crypto_sources",
     "crypto_sources_asm",
     "crypto_test_data",
-    "crypto_test_sources",
     "crypto_test_internal_headers",
+    "crypto_test_sources",
     "decrepit_internal_headers",
     "decrepit_sources",
     "decrepit_test_sources",
@@ -73,6 +73,9 @@
     hdrs = crypto_headers,
     asm_srcs = bcm_sources_asm + crypto_sources_asm,
     copts = ["-DBORINGSSL_IMPLEMENTATION"],
+    # :crypto is the bottom of the dependency tree, so it suffices to add the
+    # prefix here and propagate to users.
+    defines = ["BORINGSSL_PREFIX=bssl_bzl"],
     includes = ["include"],
     internal_hdrs = bcm_internal_headers + crypto_internal_headers,
     linkopts = select({

util/util.bzl

diff --git a/util/util.bzl b/util/util.bzl
index ff8574d..5418266 100644
--- a/util/util.bzl
+++ b/util/util.bzl
@@ -131,6 +131,7 @@
         name,
         asm_srcs = [],
         copts = [],
+        defines = [],
         deps = [],
         implementation_deps = [],
         hdrs = [],
@@ -155,6 +156,7 @@
         srcs = srcs + handle_asm_srcs(asm_srcs),
         hdrs = hdrs + internal_hdrs,
         copts = copts + boringssl_copts,
+        defines = defines,
         conlyopts = boringssl_conlyopts,
         cxxopts = boringssl_cxxopts,
         includes = includes,
@@ -173,8 +175,10 @@
             # Depend on the internal target via implementation_deps to avoid
             # re-exporting internal_hdrs.
             implementation_deps = [":" + name_internal],
-            # Although picked up transitively, re-specify deps and includes, so
-            # that targets depending on the public target also pick them up.
+            # implementation_deps suppresses transitivity of defines, deps, and
+            # includes. Respecify them so that targets depending on the public
+            # target also pick them up.
+            defines = defines,
             deps = deps,
             includes = includes,
             visibility = visibility,