base,arch: Fixed usage of `bitfield::replaceBits`

base,arch: Fixed usage of `bitfield::replaceBits`

`bitfield::replaceBits` has two parameters, `first` and `last`, which
relate to the position of the MSB and the LSB of the bits to be replaced
respectively. Therefore `first` >= `last`. In some areas of the
codebase, this assumption has been flipped with `first` <= `last`. This
caused at least one known error, recorded here:
https://gem5.atlassian.net/browse/GEM5-695. These inconsistencies have
therefore been rectified.

A note has been added to the `bitfield::replaceBits` Doxygen to make
the usage of this function clearer.

Change-Id: Ie75856161d9a5684066430ecbdcc52e04e1e77bf
Issue-on: https://gem5.atlassian.net/browse/GEM5-696
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31674
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>