runtime: cannot ReadMemStats during GC

[aclements]

GC holds the worldsema across the entire garbage collection, which means nothing else can stop the world, even during concurrent marking. Anything that does try to stop the world will block until GC is done. Probably the most interesting such operation is ReadMemStats.

We should fix GC to only hold worldsema during sweep termination and mark termination. The trickiest part of this is probably dealing with GOMAXPROCS changing during GC, which will affect mark worker scheduling.

/cc @RLH