arch-arm: RBIT instruction using mirroring func

arch-arm: RBIT instruction using mirroring func

The high speed bit-reversing function is now used
for the Aarch64/32 RBIT instruction implementation.

Change-Id: Id5a8a93d928d00fd33ec4061fbb586b8420a1c1b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5262
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>