Add cxa_thread_atexit.cpp to gn bootstrap libc++abi

Fixes this error:
ld.lld: error: undefined symbol: __cxa_thread_atexit

BUG=1047860
R=thakis

Change-Id: I0fc3ddd44b68fcdcc594a3bdc398a6b2924ac15f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075840
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745069}

tools/gn/bootstrap/libc++.ninja

@@ -62,6 +62,7 @@ build $libcxxabi/cxa_exception_storage.o: cxx_libcxxabi $buildtools/$libcxxabi/c
 build $libcxxabi/cxa_guard.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_guard.cpp
 build $libcxxabi/cxa_handlers.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_handlers.cpp
 build $libcxxabi/cxa_noexception.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_noexception.cpp
+build $libcxxabi/cxa_thread_atexit.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_thread_atexit.cpp
 build $libcxxabi/cxa_unexpected.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_unexpected.cpp
 build $libcxxabi/cxa_vector.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_vector.cpp
 build $libcxxabi/cxa_virtual.o: cxx_libcxxabi $buildtools/$libcxxabi/cxa_virtual.cpp
@@ -71,4 +72,4 @@ build $libcxxabi/stdlib_exception.o: cxx_libcxxabi $buildtools/$libcxxabi/stdlib
 build $libcxxabi/stdlib_stdexcept.o: cxx_libcxxabi $buildtools/$libcxxabi/stdlib_stdexcept.cpp
 build $libcxxabi/stdlib_typeinfo.o: cxx_libcxxabi $buildtools/$libcxxabi/stdlib_typeinfo.cpp
 
-build ../libc++.gn.so: link $libcxx/algorithm.o $libcxx/any.o $libcxx/bind.o $libcxx/charconv.o $libcxx/chrono.o $libcxx/condition_variable.o $libcxx/condition_variable_destructor.o $libcxx/debug.o $libcxx/exception.o $libcxx/functional.o $libcxx/future.o $libcxx/hash.o $libcxx/ios.o $libcxx/iostream.o $libcxx/locale.o $libcxx/memory.o $libcxx/mutex.o $libcxx/mutex_destructor.o $libcxx/new.o $libcxx/optional.o $libcxx/random.o $libcxx/regex.o $libcxx/shared_mutex.o $libcxx/stdexcept.o $libcxx/string.o $libcxx/strstream.o $libcxx/system_error.o $libcxx/thread.o $libcxx/typeinfo.o $libcxx/utility.o $libcxx/valarray.o $libcxx/variant.o $libcxx/vector.o $libcxxabi/abort_message.o $libcxxabi/cxa_aux_runtime.o $libcxxabi/cxa_default_handlers.o $libcxxabi/cxa_demangle.o $libcxxabi/cxa_exception_storage.o $libcxxabi/cxa_guard.o $libcxxabi/cxa_handlers.o $libcxxabi/cxa_noexception.o $libcxxabi/cxa_unexpected.o $libcxxabi/cxa_vector.o $libcxxabi/cxa_virtual.o $libcxxabi/fallback_malloc.o $libcxxabi/private_typeinfo.o $libcxxabi/stdlib_exception.o $libcxxabi/stdlib_stdexcept.o $libcxxabi/stdlib_typeinfo.o
+build ../libc++.gn.so: link $libcxx/algorithm.o $libcxx/any.o $libcxx/bind.o $libcxx/charconv.o $libcxx/chrono.o $libcxx/condition_variable.o $libcxx/condition_variable_destructor.o $libcxx/debug.o $libcxx/exception.o $libcxx/functional.o $libcxx/future.o $libcxx/hash.o $libcxx/ios.o $libcxx/iostream.o $libcxx/locale.o $libcxx/memory.o $libcxx/mutex.o $libcxx/mutex_destructor.o $libcxx/new.o $libcxx/optional.o $libcxx/random.o $libcxx/regex.o $libcxx/shared_mutex.o $libcxx/stdexcept.o $libcxx/string.o $libcxx/strstream.o $libcxx/system_error.o $libcxx/thread.o $libcxx/typeinfo.o $libcxx/utility.o $libcxx/valarray.o $libcxx/variant.o $libcxx/vector.o $libcxxabi/abort_message.o $libcxxabi/cxa_aux_runtime.o $libcxxabi/cxa_default_handlers.o $libcxxabi/cxa_demangle.o $libcxxabi/cxa_exception_storage.o $libcxxabi/cxa_guard.o $libcxxabi/cxa_handlers.o $libcxxabi/cxa_noexception.o $libcxxabi/cxa_thread_atexit.o $libcxxabi/cxa_unexpected.o $libcxxabi/cxa_vector.o $libcxxabi/cxa_virtual.o $libcxxabi/fallback_malloc.o $libcxxabi/private_typeinfo.o $libcxxabi/stdlib_exception.o $libcxxabi/stdlib_stdexcept.o $libcxxabi/stdlib_typeinfo.o