[C++] Rearrange code in bit-util.h/.cc for AppendWord

[asfimport]

1. See if the optimizations for AppendWord (introduced in ARROW-8413: [C++][Parquet] Refactor Generating validity bitmap for values column #6985) in the little endian case can be adapted to add parallelism to TransferBitmap.
2. Move CopyBitmap higher in the header file so it can be used for the BigEndian case.

Reporter: Micah Kornfield / @emkornfield

_{Note: This issue was originally created as ARROW-8595. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.