libc/glibc: add glibc specifics to the shared code, and use it

Final step at sharing code between glibc and eglibc.
Fall, wall of shame, fall!... :-)

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

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

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

}