mingw-w64: kernel32.a retains a reference to stage 1 GCC

[DavidEGrayson]

This causes the closure of our mingw-w64 compiler to contain the stage 1 compiler, wasting some space. Not a big deal, but it would be interesting to figure out why this is happening and it's only happening for one of the many libraries in mingw-w64.

An example of the problem:

$ /nix/store/l7f963mjahznnfjpib65wh58wgrkhwf9-binutils-2.27-i686-w64-mingw32/bin/i686-w64-mingw32-strings /nix/store/5ppzn3pqy2d7rgcfim3f7j23561wdqg8-mingw-w64-4.0.6/lib/libkernel32.a | grep stage1
/nix/store/a1x3zwii2ycdiqi8s7yfr3nc82nagm6l-gcc-5.4.0-i686-w64-mingw32-stage1/lib/gcc/i686-w64-mingw32/5.4.0/include

In general use this command to test the run-time closure:

nix-store -q --tree $(nix-build -A pkgs.i686-w64-mingw32.gcc)

Be sure to upgrade to the latest mingw-w64 before putting much effort into this.