test/radix-sort: replaced __constant input with __global const

In issue #69 it reads "The size of buffer passed as "__constant uint
*unsorted_elements" is 320 kB, whereas minimum that needs to be
supported is 64 kB." This problem has been addressed by replacing the
__constant input data with __global const input data, as changing the
input data size isn't simple.

This change also moves the original input.cl/output.cl pair to
doc/radix-sort-example as the hand-instrumented output.cl no longer
matches the input.cl. Updating output.cl manually (or with
webcl-validator) would result in the example of handling constant
address space data, which I deem as a worthwhile example to preserve.