crypto/poly1305: Add SVE2 vector-length agnostic implementation.

crypto/poly1305: Add SVE2 vector-length agnostic implementation.

Implement Poly1305 using SVE2 VLA instructions for AArch64.

This implementation is selected at runtime if SVE2 is present and the vector length is 256, 512, 1024 or 2048 bits.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/28454)