uclibc: enable Xtensa NPTL support with uClibc-ng

Contrary to mainline uClibc, uClibc-ng has support for NPTL on Xtensa,
so let's allow selecting NPTL when a uClibc version other than the
Xtensa special uClibc version is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Waldemar Brodkorb <wbx@uclibc-ng.org>