Add bt2c::reverseFixedLenIntBits()

Add bt2c::reverseFixedLenIntBits()

This new function reverses the first N least significant bits of a
64-bit integer value, sign-extending if needed, and returns the result.

I basically read [1], which shows the version of Knuth, and adapted that
code to our coding style and requirements.

[1]: https://matthewarcus.wordpress.com/2012/11/18/reversing-a-64-bit-word/

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib67fd5e8b94e0a82f46527534d47617c87013418
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12706