Add Android stlport and default toolchains to BUILD.

These are additional possibilities for --crosstool_top for Android NDK
compilation. Since the NDK doesn't have -lpthread, getting these flags
wrong leads to a linker error.

Fixes: 180084220

BUILD

@@ -69,6 +69,13 @@ create_compiler_config_setting(
     ],
 )
 
+# Android NDK builds can specify different crosstool_top flags to choose which
+# STL they use for C++. We need these multiple variants to catch all of those
+# versions of crosstool_top and reliably detect Android.
+#
+# For more info on the various crosstool_tops used by NDK Bazel builds, see:
+# https://docs.bazel.build/versions/master/android-ndk.html#configuring-the-stl
+
 config_setting(
     name = "android",
     values = {
@@ -80,6 +87,17 @@ config_setting(
     ],
 )
 
+config_setting(
+    name = "android-stlport",
+    values = {
+        "crosstool_top": "@androidndk//:toolchain-stlport",
+    },
+    visibility = [
+        # Public, but Protobuf only visibility.
+        "//:__subpackages__",
+    ],
+)
+
 config_setting(
     name = "android-libcpp",
     values = {
@@ -102,11 +120,24 @@ config_setting(
     ],
 )
 
+config_setting(
+    name = "android-default",
+    values = {
+        "crosstool_top": "@androidndk//:default_crosstool",
+    },
+    visibility = [
+        # Public, but Protobuf only visibility.
+        "//:__subpackages__",
+    ],
+)
+
 # Android and MSVC builds do not need to link in a separate pthread library.
 LINK_OPTS = select({
     ":android": [],
+    ":android-stlport": [],
     ":android-libcpp": [],
     ":android-gnu-libstdcpp": [],
+    ":android-default": [],
     ":msvc": [
         # Suppress linker warnings about files with no symbols defined.
         "-ignore:4221",