GC tweaks

[Matthew Fluet]

> If  you are really correct that the stack size can fail to be divisible by 4,
> then this is a disaster.  It would mean that if you later push an int on  the
> stack,  it  will not be aligned.  The Intel architecture will allow this, but
> it will slow you down by a huge huge amount.

Well, this same sort of thing came up in the discussion about floats on
the stack.  Objects on the stack are aligned relative to the stackTop, but
frames aren't aligned.