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.

 reporter_name="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""