Always build arm in clang bootstrap.

bootstrap is used to build libclang_rt, and without this change
libclang_rt.ios.a will not be built.

Bug: 742498
Change-Id: I8e964fc56aaa7dcec26cde5a0884a8a899e8342f
Reviewed-on: https://chromium-review.googlesource.com/575586
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488258}

tools/clang/scripts/update.py

@@ -490,7 +490,7 @@ def UpdateClang(args):
     EnsureDirExists(LLVM_BOOTSTRAP_DIR)
     os.chdir(LLVM_BOOTSTRAP_DIR)
     bootstrap_args = base_cmake_args + [
-        '-DLLVM_TARGETS_TO_BUILD=host',
+        '-DLLVM_TARGETS_TO_BUILD=X86;ARM',
         '-DCMAKE_INSTALL_PREFIX=' + LLVM_BOOTSTRAP_INSTALL_DIR,
         '-DCMAKE_C_FLAGS=' + ' '.join(cflags),
         '-DCMAKE_CXX_FLAGS=' + ' '.join(cxxflags),