windows_shared_map_cost maps 256, 16384 and 65536 pages of shared memory
(4 MiB, 256 MiB and 1 GiB) through WindowsMappings::Map, then releases
them, five times each, and prints the median map and release times, the
time per page and the number of mapped views. It only measures and
always passes, so a build of this commit and of a later one can be
compared on the same machine.
