Run valgrind on a bot

[brson]

We are not running valgrind anywhere right now, even though the linux dist builder says it's enabling valgrind, valgrind is not installed on that machine.

The makefiles probably should not silently disable valgrind like they appear to be doing.

[alexcrichton]

cc #8267
cc #8389

[brson]

Nominating because I will be super embarrassed if we release 1.0 with memory leaks.

[brson]

P-high, 1.0

[brson]

We are actually running valgrind on the snapshot bots: the basic problem here is that we have a broad suppression for most of memcheck because of #5856.

[alexcrichton]

I actually just turned valgrind off on all the bots. I recently created some new snapshot bots, and those have a very recent valgrind installed, and they uncovered a huge number of errors, most of which I don't think are relevant. I uncovered a few errors here and there, but the volume was too high to deal with and we need a new snapshot, so valgrind is actually turned off now.