Enable the __tcc_cas in the atomics.nim for tcc in windows.

[slangmgh]

The cas function for TinyC is disabled for windows in the atomics.nim, in fact it works for windows.

https://forum.nim-lang.org/t/3794/1#23666

[genotrance]

I'll be pushing some fixes in the next couple of days. 22 tests still fail though so there's more work required on ensuring tcc works correctly through and through.

[genotrance]

Updated test results with latest tcc from #head. Passes the threads tests but still 20 failing tests. Also, snip still hangs on run.

[genotrance]

Latest win32 build of tcc for convenience.