Go back to using __builtin_ctzll(). Sigh.
Go back to using __builtin_ctzll(). Sigh.

Change-Id: Ie3cee16142347c97a68877312032e0fd77d4b2ef
Reviewed-on: https://code-review.googlesource.com/c/re2/+/54370
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2