lib/math: Add a `do_div' test module

Implement a module for correctness and performance evaluation for the
`do_div' function, often handled in an optimised manner by platform
code.  Use a somewhat randomly generated set of inputs that is supposed
to be representative, using the same set of divisors twice, expressed as
a constant and as a variable each, so as to verify the implementation
for both cases should they be handled by different code execution paths.
Reference results were produced with GNU bc.

At the conclusion output the total execution time elapsed.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>