The upcoming RISC-V vector crypto extensions feature
a Zvknha extension, that provides sha256-specific instructions.
This patch provides an implementation that utilizes this
extension if available.

Tested on QEMU and no regressions observed.
Signed-off-by: NCharalampos Mitrodimas <charalampos.mitrodimas@vrull.eu>
Signed-off-by: NChristoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NHugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21923)