uclibc: NPTL thread implementation only available on MMU platforms

uclibc: NPTL thread implementation only available on MMU platforms

As indicated by the uClibc-ng source code, the NPTL thread
implementation is only available on MMU platforms, so we replicate
this dependency in Buildroot so that the appropriate thread
implementation is chosen by default on ARM noMMU.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>