crypto: ecc - Optimize vli additive operations using compiler builtins

Replace the software carry flag emulation with compiler builtins.

Even the newest compilers struggle with taking advantage of the
hardware carry flag. Compiler builtins allow the compiler to
much more easily achieve this while still remaining constant-time.

This yields an approximately 6-7% performance improvement
on the ecc_gen_privkey, ecc_make_pub_key and crypto_ecdh_shared_secret
functions on x86_64 on all curve sizes.

Additionally, the code becomes much more readable.

Signed-off-by: Fabian Blatter <fabianblatter09@gmail.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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