riscv: GCM: Simplify GCM calculation

riscv: GCM: Simplify GCM calculation

The existing GCM calculation provides some potential
for further optimizations. Let's use the demo code
from the RISC-V cryptography extension groups
(https://github.com/riscv/riscv-crypto), which represents
the extension architect's intended use of the clmul instruction.

The GCM calculation depends on bit and byte reversal.
Therefore, we use the corresponding instructions to do that
(if available at run-time).

The resulting computation becomes quite compact and passes
all tests.

Note, that a side-effect of this change is a reduced register
usage in .gmult(), which opens the door for an efficient .ghash()
implementation.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20078)