Improves the error message for invalid cpu (--cpu or --fat_apk_cpu).

Before:

ERROR: No toolchain found for cpu 'x84'.

After:

ERROR: No toolchain found for cpu 'x84'. Valid cpus are: [
  armeabi,
  armeabi-v7a,
  armeabi-v7a-hard,
  armeabi-thumb,
  armeabi-v7a-thumb,
  armeabi-v7a-hard-thumb,
  arm64-v8a,
  mips,
  mips64,
  x86,
  x86_64,
].

--
MOS_MIGRATED_REVID=105324190

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

@@ -337,12 +337,24 @@ public static CrosstoolConfig.CToolchain selectToolchain(
         break;
       }
     }
+
+    if (selectedIdentifier == null) {
+      StringBuilder cpuBuilder = new StringBuilder();
+      for (CrosstoolConfig.DefaultCpuToolchain selector : release.getDefaultToolchainList()) {
+        cpuBuilder.append("  ").append(selector.getCpu()).append(",\n");
+      }
+      throw new InvalidConfigurationException(
+          "No toolchain found for cpu '" + desiredCpu
+          + "'. Valid cpus are: [\n" + cpuBuilder + "]");
+    }
     checkToolChain(selectedIdentifier, desiredCpu);
+
     for (CrosstoolConfig.CToolchain toolchain : release.getToolchainList()) {
       if (toolchain.getToolchainIdentifier().equals(selectedIdentifier)) {
         return toolchain;
       }
     }
+
     throw new InvalidConfigurationException("Inconsistent crosstool configuration; no toolchain "
         + "corresponding to '" + selectedIdentifier + "' found for cpu '" + config.getCpu() + "'");
   }
@@ -369,14 +381,11 @@ private static void describeToolchainList(StringBuilder message,
    * spaces, letters, digits or underscores (i.e. matches the following regular expression:
    * "[a-zA-Z_][\.\- \w]*").
    *
-   * @throws InvalidConfigurationException if selectedIdentifier is null or does not match the
+   * @throws InvalidConfigurationException if selectedIdentifier does not match the
    *         aforementioned regular expression.
    */
   private static void checkToolChain(String selectedIdentifier, String cpu)
       throws InvalidConfigurationException {
-    if (selectedIdentifier == null) {
-      throw new InvalidConfigurationException("No toolchain found for cpu '" + cpu + "'");
-    }
     // If you update this regex, please do so in the javadoc comment too, and also in the
     // crosstool_config.proto file.
     String rx = "[a-zA-Z_][\\.\\- \\w]*";

src/test/java/com/google/devtools/build/lib/analysis/config/BuildConfigurationTest.java

@@ -1,4 +1,4 @@
-// Copyright 2006-2015 Google Inc. All rights reserved.
+// Copyright 2015 The Bazel Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@
 import com.google.devtools.common.options.Options;
 
 import java.util.Map;
+import java.util.regex.Pattern;
 
 /**
  * Tests for {@link BuildConfiguration}.
@@ -117,17 +118,18 @@ public void testCaching() throws Exception {
     assertEquals(a.cacheKey(), b.cacheKey());
   }
 
-  private void checkInvalidCpuError(String cpuOption, String expectedMessage) throws Exception {
+  private void checkInvalidCpuError(String cpuOption, Pattern messageRegex) throws Exception {
     try {
       create("--" + cpuOption + "=bogus");
       fail();
     } catch (InvalidConfigurationException e) {
-      assertThat(e).hasMessage(expectedMessage);
+      assertThat(e.getMessage()).matches(messageRegex);
     }
   }
 
   public void testInvalidCpu() throws Exception {
-    checkInvalidCpuError("cpu", "No toolchain found for cpu 'bogus'");
+    checkInvalidCpuError("cpu", Pattern.compile(
+        "No toolchain found for cpu 'bogus'. Valid cpus are: \\[\n(  [\\w-]+,\n)+]"));
   }
 
   public void testConfigurationsHaveUniqueOutputDirectories() throws Exception {