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 Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}