Revert #a09246191120: cc/gcc: fix C++ headers location

This was intended as a fix for g++ not finding its headers,
but it breaks in othe horrible ways. So just revert it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }