libc/uClibc: enable NPTL for snapshots

Recently, NPTL has matured a lot in uClibc, and more and more
people are interested to at least give it a try. So enable it.

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }