ART: Fix host test failure (assembler_thumb_test).

Check if "bin/{as,objcopy,objdump}" exists in the tools dir we found in "prebuilts/gcc/linux-x86/arm/".
Now there's an empty tree in platform/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9,
causing assembler_thumb_test to fail.

Change-Id: If6ba4f778b79f95239aa85e54816ab168e7d8940
diff --git a/compiler/utils/assembler_thumb_test.cc b/compiler/utils/assembler_thumb_test.cc
index 891a287..e3a9580 100644
--- a/compiler/utils/assembler_thumb_test.cc
+++ b/compiler/utils/assembler_thumb_test.cc
@@ -116,6 +116,19 @@
     std::string subdir = toolsdir + std::string("/") + std::string(entry->d_name);
     size_t eabi = subdir.find(TOOL_PREFIX);
     if (eabi != std::string::npos) {
+      // Check if "bin/{as,objcopy,objdump}" exist under this folder.
+      struct stat exec_st;
+      std::string exec_path;
+      exec_path = subdir + "/bin/" + TOOL_PREFIX + "as";
+      if (stat(exec_path.c_str(), &exec_st) != 0)
+        continue;
+      exec_path = subdir + "/bin/" + TOOL_PREFIX + "objcopy";
+      if (stat(exec_path.c_str(), &exec_st) != 0)
+        continue;
+      exec_path = subdir + "/bin/" + TOOL_PREFIX + "objdump";
+      if (stat(exec_path.c_str(), &exec_st) != 0)
+        continue;
+
       std::string suffix = subdir.substr(eabi + strlen(TOOL_PREFIX));
       double version = strtod(suffix.c_str(), nullptr);
       if (version > maxversion) {