SystemTools: move some code around that is used only on Windows-like platforms

The list of possible extensions is only filled inside an #ifdef, so the whole
usage of this list can be moved into that #ifdef's, too. The loop can be
completely avoided if it's already known that the list will not be filled at
all.

Change-Id: I89a87115babf68f9bf99d4ad523cb0851c252cd9