arch-riscv: Merge rv32 and rv64 version of xperm4 and xperm8 Remove unessential postfix like '_32' and '_64' from mnemonic Change-Id: I83d47eeccd04fe61ac8ee0addd7221abbdcefbd1 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70600 Reviewed-by: Yu-hsin Wang <yuhsingw@google.com> Tested-by: kokoro <noreply+kokoro@google.com> Maintainer: Jason Lowe-Power <power.jg@gmail.com>