Resolves: #15424

The compiler would optimize the benchmark function call out of the
loop and call it only once, resulting in blazingly fast times for some
benchmarks (notably atan, sin and cos).  Mark the inputs as volatile
so that the code is forced to read again from the input for each
iteration.

Some math functions have distinct performance characteristics in
specific domains of inputs, where some inputs return via a fast path
while other inputs require multiple precision calculations, that too
at different precision levels.  The way to implement different domains
was to have a separate source file and benchmark definition, resulting
in separate programs.

This clutters up the benchmark, so this change allows these domains to
be consolidated into the same input file.  To do this, the input file
format is now enhanced to allow comments with a preceding # and
directives with two # at the begining of a line.  A directive that
looks like:

tells the benchmark generation script that what follows is a different
domain of inputs.  The value of the 'name' directive (in this case,
foo) is used in the output.  The two input domains are then executed
sequentially and their results collated separately.  with the above
directive, there would be two lines in the result that look like:

func(): ....
func(foo): ...

The idea to run benchmarks for a constant number of iterations is
problematic.  While the benchmarks may run for 10 seconds on x86_64,
they could run for about 30 seconds on powerpc and worse, over 3
minutes on arm.  Besides that, adding a new benchmark is cumbersome
since one needs to find out the number of iterations needed for a
sufficient runtime.

A better idea would be to run each benchmark for a specific amount of
time.  This patch does just that.  The run time defaults to 10 seconds
and it is configurable at command line:

  make BENCH_DURATION=5 bench

See benchtests/Makefile to know how to use it.