riscv: GCM: Use riscv.pm

riscv: GCM: Use riscv.pm

A recent commit introduced a Perl module for common code.
This patch changes the GCM code to use this module, removes duplicated code,
and moves the instruction encoding functions into the module.

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)