Remove --disable-tcache.

Simplify configuration by removing the --disable-tcache option, but
replace the testing for that configuration with
--with-malloc-conf=tcache:false.

Fix the thread.arena and thread.tcache.flush mallctls to work correctly
if tcache is disabled.

This partially resolves #580.

scripts/gen_run_tests.py

@@ -18,7 +18,7 @@ possible_config_opts = [
     '--enable-debug',
     '--enable-prof',
     '--disable-stats',
-    '--disable-tcache',
+    '--with-malloc-conf=tcache:false',
 ]
 
 print 'set -e'