Revert "Change cross host from i686-w64-mingw32 to x86_64-w64-mingw32"

This reverts commit 7ccac016591f0d941f10fac0e044ac8ca6daf9da.

Silly me, I just installed the wrong tools. x86_64 can't yet be used for
cross compiling, see luislavena/rake-compiler#95.
Explicitly specify platform for cross task

See luislavena/rake-compiler#95.
Fix cross compilation tasks

Something changed with rake-compiler so that the file now needs to be in
a task. Restructure the tasks to do that, which is actually nicer.
Remove -static-libgcc -static-libstdc++ for cross again

The resulting gem was 18 MB with these, removing them makes it go back
to 2.6 MB again. Something changed somewhere :).