base: Function for mirroring bits in variable length word

This patch introduces a high-speed template function for mirroring the
bits (MSB=>LSB) in a variable length word. The function is achieving
high performances since it is using a look-up table.

Change-Id: Ib0d0480e68d902f25655f74d243de305103eff75
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/5261
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

src/base/SConscript

@@ -34,6 +34,7 @@ if env['CP_ANNOTATE']:
     SimObject('CPA.py')
     Source('cp_annotate.cc')
 Source('atomicio.cc')
+Source('bitfield.cc')
 Source('bigint.cc')
 Source('bitmap.cc')
 Source('callback.cc')