[llvm-symbolizer] Fix coff-dwarf.test

This was a bug in the test that was only exposed as a result of
refactoring some code in lit configuration files.  Previously,
llvm's lit configuration would only set the target-windows feature
if the system was also windows.  Since cross-compilation is
a thing, this isn't correct.  target-windows should be set
independently of system-windows.

Adding to that bug, this particular test then checked for
target-windows when it really meant "can I call a certain API on
the host machine", which is what system-windows is for.

Ultimately, this test only works if *both* the target and host
are Windows, so I've updated the test to reflect that.

llvm-svn: 313468