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 M68k-specific values

 CT_DoArchTupleValues() {

     :

 }