gio: add a simple gpoll performance test for W32

It just creates a number of socket pairs, then triggers read-ready status on these pairs in different patterns (none, one, half, all) and checks how much time it takes to g_poll() those. Also sometimes posts a Windows message and polls for its arrival.
The g_main_context_new() is necessary to initialize g_poll() debugging on W32.

Measures minimal and maximal time it takes to g_poll(), as well as the average, over 1000 runs.
Collects the time values into 25 non-linear buckets between 0ns and 1000ns, and displays them at the conclusion of each subtest.

https://bugzilla.gnome.org/show_bug.cgi?id=785468