A stack size of 128k is too small for rhel6/ppc64. Upping to 256k.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5120 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishtest/tests/c00031.vtc

@@ -9,7 +9,7 @@ server s1 {
 	txresp
 } -start
 
-varnish v1 -arg "-p thread_pool_stack=131072" -vcl+backend {} -start
+varnish v1 -arg "-p thread_pool_stack=262144" -vcl+backend {} -start
 
 client c1 {
 	txreq -url "/"