Clang update script: set CMAKE_ASM_FLAGS when building compiler-rt

After compiler-rt r344751, CMAKE_CXX_FLAGS doesn't get passed to all
objects.

TBR=thakis

Bug: 897627
Change-Id: Iea0eda192c6da8d6d662732e7ad456e0ba3c0b46
Reviewed-on: https://chromium-review.googlesource.com/c/1292883Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601523}

tools/clang/scripts/update.py

@@ -802,6 +802,7 @@ def UpdateClang(args):
         '-DLLVM_CONFIG_PATH=' + os.path.join(LLVM_BUILD_DIR, 'bin/llvm-config'),
         '-DCMAKE_C_FLAGS=' + ' '.join(cflags),
         '-DCMAKE_CXX_FLAGS=' + ' '.join(cflags),
+        '-DCMAKE_ASM_FLAGS=' + ' '.join(cflags),
         '-DSANITIZER_CXX_ABI=none',
         '-DSANITIZER_CXX_ABI_LIBRARY=' + abi_libs,
         '-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-u__cxa_demangle',