[v2] linux-user: bigger default stack

PTHREAD_STACK_MIN (16KB) is somewhat inadequate for a new stack for new
QEMU threads. Set new limit to 256K which should be enough, yet doesn't
increase memory pressure significantly.

Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>
