misc: Rename BitfieldBackend namespace as bitfield_backend
misc: Rename BitfieldBackend namespace as bitfield_backend

As part of recent decisions regarding namespace
naming conventions, all namespaces will be changed
to snake case.

::BitfieldBackend became ::bitfield_backend.

Change-Id: Ibf6c5a93baa9b07aab84dc4ee98960a57d925eaf
Signed-off-by: Daniel R. Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45425
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>

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