fixes for -l usage on MSWin32 and cygwin

This allows you to find the DLL from an import library on MSWin32 and cygwin.

It may pay to put `_dl_findfile` and its dependencies (all core at least) into a FFI::Sweet::Win32 so that stuff doesn't need to be loaded on platforms that do not require the extra logic.

On MSWin32 this will may (probably?) will only work with Perl built from GCC.  Borland and MS are also technically supported, but I don't know how widely they are used.

Also won't work if there is no .la file and the user doesn't have ar installed.  I looked at using Archive::Ar, but it didn't seem quite right in the head.