Fix support for windows atomics

Make CRYPTO_atomic_add consistent with
CRYPTO_atomic_load_int and set the
reader_idx under write_lock since there
is no CRYPTO_atomic_store_int.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26963)