uClibc: Deactivate oversized thread stack cache

A small system like the common home router doesn't have 40 MiB per process for
a dirty stack cache. This can easily lead to an overbooking OOM problem and
caused a lot of hangs+reboots on 32 MiB systems running nodogsplash.

Not using a stack cache can increase the time to spawn new threads. This is
hopefully no problem for system not running a lot of parallel computations.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38851 3c298f89-4303-0410-b956-a3cf2f4a3e73