debug/gdb: allow static build with complibs

It is now possible to built static versions of the companion libraries,
so we can now build a static cross-gdb using the complibs.

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

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

 }