Fix Android build errors on macOS.

- Add sysroot path to environment variable 'CPPFLAGS':
  When checking C preprocessor, the 'configure' script of libbacktrace uses 'CPPFLAGS' rather than 'CFLAGS' and doesn't get the correct search path. (#15758)
  This check passes on Linux because the '/lib/cpp' fallback is available there.

- Introduce CMake toolchain file for Android cross compiling:
  CMake needs several variables [1] to cross compile for Android.
  It works (accidentally) on Linux because cmake-rs sets compilers correctly and binutils for Linux & Android are pretty much the same.

[1] https://cmake.org/cmake/help/v3.7/manual/cmake-toolchains.7.html#cross-compiling-for-android-with-the-ndk

python/servo/build_commands.py

@@ -306,6 +306,9 @@ class MachCommands(CommandBase):
                 "-I" + cxx_include,
                 "-I" + cxxabi_include])
             env["NDK_ANDROID_VERSION"] = android_platform.replace("android-", "")
+            env['CPPFLAGS'] = ' '.join(["--sysroot", env['ANDROID_SYSROOT']])
+            env["CMAKE_ANDROID_ARCH_ABI"] = self.config["android"]["lib"]
+            env["CMAKE_TOOLCHAIN_FILE"] = path.join(self.android_support_dir(), "toolchain.cmake")
 
         cargo_binary = "cargo" + BIN_SUFFIX