luajit: Fix cross to different bitness from Cygwin
luajit: Fix cross to different bitness from Cygwin

luajit only supports cross compilation to architectures with the same
pointer width. This is hard on Cygwin, since it has no multilib
compilers, but luckily the target compilers can be used instead.

chhe/mingw-w64-cmake,lachs0r/mingw-w64-cmake