Rework the naked C-variadic example

For this example, rather than showing raw compiler output, which
contains a spill-and-reload sequence that is hard to follow, let's
write some assembly by hand.  And let's pick an example that motivates
the use of variadics.  We'll compute the dot product of a vector with
a projected scalar.  We'll support vector dimensions up to what can be
passed in with the `xmm` registers.