libc: hide no-thread for those C libraries that do not support it

Building non-threaded glibc has been unsupported for a long time, now:
http://sourceware.org/ml/libc-alpha/2005-08/msg00091.html

As eglibc is a spin-off of glibc: ditto.

So do not offer that possibility in the menuconfig.
Thanks to Thomas Petazzoni for spotting, and helping to solve, the issue!

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

 COPYING