libc/eglibc: move generic code to a common file

The build procedure for eglibc is generic enough to
be shared between glibc and eglibc. This includes:
- headers install (empty!)
- start files build
- complete libc build
- libc finish (empty!)
- add-ons list

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

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }