sigaltstack and cygwin

[Stephen Weeks]

> Fixed.  It was really easy and I should have remembered it earlier.  There
> is a hack in the register allocator to _not_ adjust c_stackP after the
> last C call in a basic block; 
...

Great.  Thanks.

> Results are mixed.  Reserving esp can both hurt or help; it hurts just
> from register pressure.  It can help when there are a lot of C calls
> (wc-input1) and the cost of fetching c_stackP is the bottleneck.

It hurt a lot less than I would have expected.

I wasn't aware of the tensor problem.  I am investigating.