Set a default value for --host_crosstool_top.

Previously it was unset and defaulted to --crosstool_top, but that
doesn't make sense for android/apple/etc builds.

Ideally we would set --host_crosstool_top to the value of
--crosstool_top at the beginning of the build, but that isn't currently
possible.

src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java

@@ -560,21 +560,35 @@ public Label getFdoPrefetchHintsLabel() {
               + "except bar.o.")
   public List<PerLabelOptions> perFileLtoBackendOpts;
 
+  /** See {@link #hostCrosstoolTop} documentation. * */
+  private static final String HOST_CROSSTOOL_TOP_NOT_YET_SET = "HOST CROSSTOOL TOP NOT YET SET";
+
+  /**
+   * The value of "--crosstool_top" to use for building tools.
+   *
+   * <p>We want to make sure this stays bound to the top-level configuration when not explicitly set
+   * (as opposed to a configuration that comes out of a transition). Otherwise we risk using the
+   * wrong crosstool (i.e., trying to build tools with an Android-specific crosstool).
+   *
+   * <p>To accomplish this, we initialize this to a special value that means "I haven't been set
+   * yet" and use {@link #getNormalized} to rewrite it to {@link #hostCrosstoolTop} <b>only</b> from
+   * that default. Blaze always evaluates top-level configurations first, so they'll trigger this.
+   * But no followup transitions can.
+   */
   @Option(
-    name = "host_crosstool_top",
-    defaultValue = "null",
-    converter = LabelConverter.class,
-    documentationCategory = OptionDocumentationCategory.TOOLCHAIN,
-    effectTags = {
-      OptionEffectTag.LOADING_AND_ANALYSIS,
-      OptionEffectTag.CHANGES_INPUTS,
-      OptionEffectTag.AFFECTS_OUTPUTS
-    },
-    help =
-        "By default, the --crosstool_top and --compiler options are also used "
-            + "for the host configuration. If this flag is provided, Bazel uses the default libc "
-            + "and compiler for the given crosstool_top."
-  )
+      name = "host_crosstool_top",
+      defaultValue = HOST_CROSSTOOL_TOP_NOT_YET_SET,
+      converter = LabelConverter.class,
+      documentationCategory = OptionDocumentationCategory.TOOLCHAIN,
+      effectTags = {
+        OptionEffectTag.LOADING_AND_ANALYSIS,
+        OptionEffectTag.CHANGES_INPUTS,
+        OptionEffectTag.AFFECTS_OUTPUTS
+      },
+      help =
+          "By default, the --crosstool_top and --compiler options are also used "
+              + "for the host configuration. If this flag is provided, Bazel uses the default libc "
+              + "and compiler for the given crosstool_top.")
   public Label hostCrosstoolTop;
 
   @Option(
@@ -1004,10 +1018,20 @@ public Label getFdoPrefetchHintsLabel() {
   /** See {@link #targetLibcTopLabel} documentation. * */
   @Override
   public FragmentOptions getNormalized() {
+    CppOptions newOptions = (CppOptions) this.clone();
+    boolean changed = false;
     if (targetLibcTopLabel != null
         && targetLibcTopLabel.getName().equals(TARGET_LIBC_TOP_NOT_YET_SET)) {
-      CppOptions newOptions = (CppOptions) this.clone();
       newOptions.targetLibcTopLabel = libcTopLabel;
+      changed = true;
+    }
+    if (hostCrosstoolTop != null
+        && hostCrosstoolTop.getName().equals(HOST_CROSSTOOL_TOP_NOT_YET_SET)) {
+      // Default to the initial target crosstoolTop.
+      newOptions.hostCrosstoolTop = crosstoolTop;
+      changed = true;
+    }
+    if (changed) {
       return newOptions;
     }
     return this;
@@ -1017,14 +1041,8 @@ public FragmentOptions getNormalized() {
   public FragmentOptions getHost() {
     CppOptions host = (CppOptions) getDefault();
 
-    // The crosstool options are partially copied from the target configuration.
-    if (hostCrosstoolTop == null) {
-      host.cppCompiler = cppCompiler;
-      host.crosstoolTop = crosstoolTop;
-    } else {
-      host.crosstoolTop = hostCrosstoolTop;
-      host.cppCompiler = hostCppCompiler;
-    }
+    host.crosstoolTop = hostCrosstoolTop;
+    host.cppCompiler = hostCppCompiler;
 
     // hostLibcTop doesn't default to the target's libcTop.
     // Only an explicit command-line option will change it.