riscv: sha256: Provide a Zvknha-based implementation

riscv: sha256: Provide a Zvknha-based implementation

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: Charalampos Mitrodimas <charalampos.mitrodimas@vrull.eu>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21923)