riscv: Add RISC-V Vector opcode in riscv.pm

riscv: Add RISC-V Vector opcode in riscv.pm

Added helper functions and opcode encoding functions
in riscv.pm perl module to avoid pointless code duplication.

Signed-off-by: Phoebe Chen <phoebe.chen@sifive.com>
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)