Cherry-pick "Do not override clang compiler internal builtins" in ICU

Merge
https://github.com/unicode-org/icu/pull/770

This is required to avoid breaking builtins detection in chromium code
that includes ICU.

Bug: 819294
Change-Id: Ia4c8978fc30131903de50be6995ff5aebad5c1e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1716752


Reviewed-by: Jungshik Shin <jshin@chromium.org>