Merge to upstream r347903.

Test: external/libcxx/run_tests.py --bitness 32
Test: external/libcxx/run_tests.py --bitness 64
Test: external/libcxx/run_tests.py --bitness 64 --host
Bug: None
Change-Id: I66f48297a65826cc90e8254355e75d5c85ff2bff