Fall back to std::shared_mutex, not std::mutex.

Windows users have SRWLOCK. POSIX users have pthread_rwlock_t. I
don't know who else is out there, but they should hopefully have
std::shared_mutex in 2022. (If not, I'm sure they will tell me.)

Change-Id: Ica0def49b0cc58119aa9587f47872ee497d03de3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59890
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>