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>

 COPYING