Fix up mingw64 target.
mk: Get rid of redundant LIBUV_FLAGS.
auto merge of #10578 : luqmana/rust/mingw64, r=alexcrichton

With these changes I was able to cross compile for windows from a linux box. (Using the mingw-w64 package on Debian Testing).

Fixed a bug where the `target_family` cfg would be wrong when targeting something with a different value than the host. (i.e windows -> unix or unix -> windows).

Also, removed `LIBUV_FLAGS` in `mk/rt.mk` because of the redundancy between it and `CFG_GCCISH_CFLAGS_(target)`.

After this we can create a snapshot and migrate to mingw64 instead of mingw32.