Go back to using __builtin_ctzll(). Sigh.

Change-Id: Ie3cee16142347c97a68877312032e0fd77d4b2ef
Reviewed-on: https://code-review.googlesource.com/c/re2/+/54370
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit b4073a9d29cdf7be063b8daaa9c8dca5fde2400d)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/54371