Cherry-pick "Remove obsolete U_HAVE_STD_ATOMICS and similar @internal macros." in ICU

Merge
https://github.com/unicode-org/icu/pull/712

This is a previous step to land the patch that replaces the way ICU declares the
clang builtins, that breaks detection of builtins in chromium code. Applying this patch
we drop code that would depend on the updated builtin macros.

Bug: 819294
Change-Id: I02935578554a77acae4073ccbd6bd9e21d5eb1be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1778021
Reviewed-by: Jungshik Shin <jshin@chromium.org>
3 files changed