libc/glibc: only install start files for NPTL

Building the start files requires a shared-capable compiler, which we do
not have when the threading implementation is LinuxThreads.

So, only build the start files when the threading implementations is NPTL.

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

 #!/bin/sh

 call_to=@@DEFAULT_LD@@

 case "${CTNG_LD_IS}" in

     bfd)    call_to=bfd;;

     gold)   call_to=gold;;

 esac

 exec "${0}.${call_to}" "$@"

 exit $?