Fix ARM64 detection.
Fix ARM64 detection.

Use only __aarch64__ and don't look for __arm64__ at all.
It turns out that clang defines both and GCC only the former.
Hence, looking only for __aarch64__ should be safe.

BUG=chromium:354405,chromium:358092
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/10939004

Patch from Primiano Tucci <primiano@chromium.org>.

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@5821 4adac7df-926f-26a2-2b94-8c16560cd09d

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