Disable atomic refcounts with no-threads

Disable atomic refcounts with no-threads

This is needed for building with '-march=i386 no-threads', on platforms
where libatomic is not available (djgpp, specifically).  The
implementation now falls back to 'CRYPTO_atomic_add()', which performs
plain lock-free addition in a 'no-threads' build.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19751)