kernel/mingw32: use pre-set headers dir

There is a variable that contains the headers directory,
so there's no need to re-compute it again.

[Yann E. MORIN: split the original patch]

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }